[FLINK-35000][build] Updates link to test code convention in pull request template

[XComp]

What is the purpose of the change

The website update changed the anchor format causing the PullRequest template link to not point to the right location.

Brief change log

• Updates link in PR template

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

[RyanSkraba]

Thanks, this is the correct new anchor.