[FLINK-18591][metrics][documentation] Fix the format issue for metrics web page

[zhijiangW]

What is the purpose of the change

Fix the format issue for metrics web page in link https://ci.apache.org/projects/flink/flink-docs-release-1.11/monitoring/metrics.html#checkpointing

Brief change log

Fix the metrics.md for adjusting the format.

Verifying this change

build the docs locally to verify the metrics web page.

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/Mesos, 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)

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Automated Checks

Last check on commit d06c015 (Tue Jul 14 03:49:02 UTC 2020)

Warnings:

• Documentation files were touched, but no .zh.md files: Update Chinese documentation or file Jira ticket.

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• ❓ 1. The [description] looks good.
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• ❓ 3. Needs [attention] from.
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• ❓ 5. Overall code [quality] is good.

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CI report:

• d06c015 Azure: SUCCESS

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[zentol]

+1

[zhijiangW]

Thanks for review @zentol , merging!