Fixed CONTRIBUTING.md markdown lints (flutter/engine#50439)

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[C++, Objective-C, Java style guides]:
https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
[testing the engine]:
https://github.com/flutter/flutter/wiki/Testing-the-engine
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat

engine/src/flutter/CONTRIBUTING.md

@@ -1,10 +1,8 @@
-Contributing to the Flutter engine
-==================================
+# Contributing to the Flutter engine
 
 _See also: [Flutter's code of conduct][code_of_conduct]_
 
-Welcome
--------
+## Welcome
 
 For an introduction to contributing to Flutter, see [our contributor
 guide][contrib_guide].
@@ -14,25 +12,25 @@ the Engine development environment][engine_dev_setup] on our wiki. Those
 instructions are part of the broader onboarding instructions described in the
 contributing guide.
 
-### Style
+## Style
 
 The Flutter engine follows Google style for the languages it uses:
+
 - [C++](https://google.github.io/styleguide/cppguide.html)
   - **Note**: The Linux embedding generally follows idiomatic GObject-based C
     style. Use of C++ is discouraged in that embedding to avoid creating hybrid
     code that feels unfamiliar to either developers used to working with
     `GObject` or C++ developers. For example, do not use STL collections or
     `std::string`. Exceptions:
-      - C-style casts are forbidden; use C++ casts.
-      - Use `nullptr` rather than `NULL`.
-      - Avoid `#define`; for internal constants use `static constexpr` instead.
+    - C-style casts are forbidden; use C++ casts.
+    - Use `nullptr` rather than `NULL`.
+    - Avoid `#define`; for internal constants use `static constexpr` instead.
 - [Objective-C][objc_style] (including [Objective-C++][objcc_style])
 - [Java][java_style]
 
 C/C++ and Objective-C/C++ files are formatted with `clang-format`, and GN files
 with `gn format`.
 
-[build_status]: https://cirrus-ci.com/github/flutter/engine
 [code_of_conduct]: https://github.com/flutter/flutter/blob/master/CODE_OF_CONDUCT.md
 [contrib_guide]: https://github.com/flutter/flutter/blob/master/CONTRIBUTING.md
 [engine_dev_setup]: https://github.com/flutter/flutter/wiki/Setting-up-the-Engine-development-environment
@@ -40,8 +38,7 @@ with `gn format`.
 [objcc_style]: https://google.github.io/styleguide/objcguide.html#objective-c
 [java_style]: https://google.github.io/styleguide/javaguide.html
 
-
-### Fuchsia Contributions from Googlers
+## Fuchsia Contributions from Googlers
 
 Googlers contributing to Fuchsia should follow the additional steps at:
 go/flutter-fuchsia-pr-policy.