Fix the docs:lint rake task

What does this MR do?

Spotted in https://gitlab.com/gitlab-org/gitlab/-/pipelines/188289214

rake docs:lint complains if you use rake instead of Rake, but the job is currently set to allowed-to-fail in some circumstances (pipelines against master?), so it got merged anyway.

Might as well fix it 😅

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #244848 (closed)