Fix ping-incorrect-review-label policy

What does this MR do and why?

The ping-incorrect-review-label has failed to assign a reviewer because last_reviewer returns a username and select_random_merge_request_coach a mention. This can result in /assign_reviewer @@user in the final comment which fails to actually assign the reviewer.

More context and discussion here: https://canary.discord.com/channels/778180511088640070/997442331202564176/1055884800755515424

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review