[hotfix] Adds Override annotation

[luyizhizaio]

What is the purpose of the change

This PR proposes to add @Override for getSelfGateway() method in PekkoRpcService. Since this method is from RpcService interface.

Brief change log

Adds @Override for PekkoRpcService.getSelfGateway() method.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)

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CI report:

• 63d92f03a35f6ecd94b389abc728750aa4763714 Azure: FAILURE

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