Resolve "Transient failure in spec/features/merge_request/user_posts_notes_spec.rb:156"

What does this MR do?

Loosens the assertion on feature spec spec/features/merge_request/user_posts_notes_spec.rb:156 to reduce flakiness.

The test is checking that an updated note could be updated again with markdown buttons. It should be sufficient to check that the new note is saved.

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

Closes #118803 (closed)