[FLINK-20069][build] Fix docs_404_check

[dianfu]

What is the purpose of the change

This pull request fixes the docs_404_check

Verifying this change

Verified in my personal azure pipeline that it takes effect:
Failed if there are broken links in the documentation: https://dev.azure.com/dianfu/Flink/_build/results?buildId=237&view=logs&j=64eb2d20-7d61-5c6d-9ae0-2f5940b1051d&t=36ef8439-438c-5ac6-2742-d7c6b595805a
Succeed based on #14009 which fixed the broken links: https://dev.azure.com/dianfu/Flink/_build/results?buildId=236&view=logs&j=64eb2d20-7d61-5c6d-9ae0-2f5940b1051d&t=36ef8439-438c-5ac6-2742-d7c6b595805a

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/Mesos, 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 applicableno)

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Automated Checks

Last check on commit c52d50a (Tue Nov 10 11:55:34 UTC 2020)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

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CI report:

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[sjwiesman]

+1 thank you for addressing this. It doesn't look like there is a 1.12 branch yet so let's try and get this in. I suspect we'll be backporting a number of docs prs between now and the release.

[rmetzger]

We've decided to fork off the "release-1.12" branch once master is considered fairly stable, to avoid this backporting all the time.

[rmetzger]

Thanks a lot for this fix! Merging ...