[FLINK-10639][docs] Fix java syntax error in document

[hequn8128]

What is the purpose of the change

Fix java syntax error in document. For example, the StreamTableSourceFactory is a trait. So the java example in the document should using implements keyword.

Brief change log

• Use implements keyword instead of extends for java in Flink docs.

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, Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

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

[sunjincheng121]

Thanks for the PR @hequn8128 ！ Merging ...
Best,
Jincheng