[FLINK-14866][doc] fix a few broken doc links

[bowenli86]

What is the purpose of the change

fix a few broken doc links

Brief change log

fix a few broken doc links

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:

N/A

Documentation

N/A

[bowenli86]

@dianfu @xuefuz @lirui-apache @zjuwangg

should have caught all the remaining broken links of function docs

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Last check on commit cc6fc68 (Wed Apr 15 11:37:40 UTC 2020)

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[xuefuz]

LGTM.

[dianfu]

@bowenli86 Thanks for the quick fix. LGTM.