[hotfix][annotation] Refine java doc for annotation VisibleForTesting

[luoyuxia]

What is the purpose of the change

To refine the java doc for the annotation VisibleForTesting

Brief change log

• Change This annotations to This annotation

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
• If yes, how is the feature documented? N/A

[flinkbot]

CI report:

• 63685a365478af0b17a0efca721347214ca6087e Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[luoyuxia]

@MartijnVisser Do you think this change makes sense?

[MartijnVisser]

@luoyuxia To be honest, I see very little value in this since it's a small but obvious grammar error. To just get this merged, will consume the CI for quite some time, which I think is more valuable to be spent on other PRs.