Correcting the typo of Flutter in projects (#6850)

In part of the documentation, Flutter is spelled out with a typo. I fixed it using a repository search.

- [] The title of the PR starts with the name of the package surrounded by square brackets, e.g. `[shared_preferences]`

script/tool/lib/src/pubspec_check_command.dart

@@ -473,7 +473,7 @@ class PubspecCheckCommand extends PackageLoopingCommand {
     Version? minMinFlutterVersion,
   }) {
     String unknownDartVersionError(Version flutterVersion) {
-      return 'Dart SDK version for Fluter SDK version '
+      return 'Dart SDK version for Flutter SDK version '
           '$flutterVersion is unknown. '
           'Please update the map for getDartSdkForFlutterSdk with the '
           'corresponding Dart version.';