[hotfix][docs] Makes the japicmp script documentation more consistent after the change in cb442936.

[XComp]

What is the purpose of the change

PR #21184 updated the documentation a bit leaving it in a kind of inconsistent state. This hotfix PR fixes this issue. Reading the previous version of the comment was confusing because of the snapshot version being used for specifying the reference version but also for refering to the snapshot branch. I tried to make this more explicit now with that PR.

Brief change log

• Updates docs in script

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:

• 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, 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 applicable

[flinkbot]

CI report:

• 76b3d54 Azure: FAILURE

Bot commands

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• @flinkbot run azure re-run the last Azure build

[XComp]

Thanks, @zentol . The build failure is due to FLINK-24119. I'm merging without rerunning it because it's just docs update.