[hotfix][doc] Use the flink-connector-jdbc v3.1 branch docs

[wanglijie95]

What is the purpose of the change

Currently, the lastest stable flink-connector-jdbc branch is v3.1, we should use the doc in that branch.

Verifying this change

Only change docs, no tests needed.

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? (not applicable)

[flinkbot]

CI report:

• 11d2bd3 Azure: SUCCESS

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