Update Gradle plugin upgrade guide (#10201)

This PR makes a tiny adjustement to [Deprecated imperative apply of
Flutter's Gradle
plugins](https://docs.flutter.dev/release/breaking-changes/flutter-gradle-plugin-apply)
migration guide.

This change brings the suggested `settings.gradle` in line with
`settings.gradle` from the latest generated template:
https://github.com/flutter/flutter/blob/1e8dd1e4d6d70c5e06525bea3fb164a03d7a6c1d/packages/flutter_tools/templates/app_shared/android.tmpl/settings.gradle.tmpl

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- [x] This PR doesn’t contain automatically generated corrections
(Grammarly or similar).
- [x] This PR follows the [Google Developer Documentation Style
Guidelines](https://developers.google.com/style) — for example, it
doesn’t use _i.e._ or _e.g._, and it avoids _I_ and _we_ (first person).
- [x] This PR uses [semantic line
breaks](https://github.com/dart-lang/site-shared/blob/main/doc/writing-for-dart-and-flutter-websites.md#semantic-line-breaks)
of 80 characters or fewer.

src/release/breaking-changes/flutter-gradle-plugin-apply.md

@@ -44,10 +44,9 @@ pluginManagement {
  def flutterSdkPath = properties.getProperty("flutter.sdk")
  assert flutterSdkPath != null, "flutter.sdk not set in local.properties"
  return flutterSdkPath
- }
- settings.ext.flutterSdkPath = flutterSdkPath()
+ }()
 
- includeBuild("${settings.ext.flutterSdkPath}/packages/flutter_tools/gradle")
+ includeBuild("$flutterSdkPath/packages/flutter_tools/gradle")
 
  repositories {
  google()