Fixed `AsyncValue.guard` documentation

[drown0315]

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

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• [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.

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Latest commit: 2774a9bc91339d8b60ff2cd1d15bebedeeb14fa8

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