[FLINK-29920][doc] Minor reformat Kafka connector documentation

[liuml07]

What is the purpose of the change

We used some HTML tag in the documentation which does not interpret Markdown format nicely. This fixes this by replacing with Markdown tags.

Brief change log

Before

After

Verifying this change

We used some HTML tag in the documentation which does not interpret Markdown format nicely. This fixes this by replacing with Markdown tags.

(Please pick either of the following options)

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): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• 1370b8b Azure: FAILURE

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[liuml07]

CC: @wuchong @danny0405

[wuchong]

Thanks for the contribution. LGTM.