Fixed `AsyncValue.guard` documentation

Fixed the usage of asyncValue.guard in the documentation

Before:

  ///   Future<void> sideEffect() async {
  ///     state = const AsyncValue.loading();
  ///     try {
  ///       final response = await dio.get('my_api/data');
  ///       final data = MyData.fromJson(response);
  ///       state = AsyncValue.data(data);
  ///     } catch (err, stack) {
  ///       state = AsyncValue.error(err, stack);
  ///     }
  ///   }
  /// }

After:

  ///   Future<void> sideEffect() async {
  ///     state = const AsyncValue.loading();
  ///     try {
  ///       final response = await dio.get('my_api/data');
  ///       final data = MyData.fromJson(response);
  ///       if(mounted){
  ///          state = AsyncValue.data(data);
  ///       }
  ///     } catch (err, stack) {
  ///       if(mounted){
  ///          state = AsyncValue.error(err, stack);
  ///       }
  ///     }
  ///   }
  /// }

Because provider may be disposed after await, add code to check if mounted.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).

[x] I have updated the CHANGELOG.md of the relevant packages. Changelog files must be edited under the form:
```
## Unreleased fix/major/minor

- Description of your change. (thanks to @yourGithubId)
```
[x] If this contains new features or behavior changes, I have updated the documentation to match those changes.

rrousselGit / riverpod

Fixed `AsyncValue.guard` documentation #3440

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