[hotfix] Javadoc should end the first sentence with a period

[aniaan]

This is a simple JavaDoc fix, some of the problems found when checkstyle

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Last check on commit 3a1e649 (Sat Aug 28 11:16:53 UTC 2021)

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[zentol]

Some parts of flink are not covered by checkstyle for historical reasons, this class being one of them. Closing the PR because we have no actual plans to rectify this.