Update matchesGoldenFile documentation reference to goldenFileComparator (#150343)

Updates the documentation for the [`matchesGoldenFile`](https://api.flutter.dev/flutter/flutter_test/matchesGoldenFile.html) comparator. 

The documentation currently references the abstract class [`GoldenFileComparator`](https://api.flutter.dev/flutter/flutter_test/GoldenFileComparator-class.html) "as the backend for this matcher", however I believe it should instead reference to [`goldenFileComparator`](https://api.flutter.dev/flutter/flutter_test/goldenFileComparator.html) that also clearly states in its documentation that it is "This comparator is used as the backend for [matchesGoldenFile](https://api.flutter.dev/flutter/flutter_test/matchesGoldenFile.html).".

Knowledge about [`goldenFileComparator`](https://api.flutter.dev/flutter/flutter_test/goldenFileComparator.html) is useful for Flutter developers since it is required when trying to set their own custom comparator, and there is currently no reference to it within the [`matchesGoldenFile`](https://api.flutter.dev/flutter/flutter_test/matchesGoldenFile.html). The abstract class [`GoldenFileComparator`](https://api.flutter.dev/flutter/flutter_test/GoldenFileComparator-class.html) is still being referenced within the second suggestion.

#76337, precisely flutter/flutter#76337 (comment)

Author: alestiago

packages/flutter_test/lib/src/matchers.dart

@@ -538,7 +538,7 @@ Matcher coversSameAreaAs(Path expectedPath, { required Rect areaToCompare, int s
 ///
 /// See also:
 ///
-///  * [GoldenFileComparator], which acts as the backend for this matcher.
+///  * [goldenFileComparator], which acts as the backend for this matcher.
 ///  * [LocalFileComparator], which is the default [GoldenFileComparator]
 ///    implementation for `flutter test`.
 ///  * [matchesReferenceImage], which should be used instead if you want to