Fix typo in ButtonBar documentation (#177078)

Changed "replace" to "replaced"

_[ButtonBar] has been **replaced** by a more efficient widget,
[OverflowBar]._

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packages/flutter/lib/src/material/button_bar.dart

@@ -22,7 +22,7 @@ import 'dialog.dart';
 ///
 /// ## Updating to [OverflowBar]
 ///
-/// [ButtonBar] has been replace by a more efficient widget, [OverflowBar].
+/// [ButtonBar] has been replaced by a more efficient widget, [OverflowBar].
 ///
 /// ```dart
 /// // Before