Fix help text in Admin Area > Background Jobs

What does this MR do?

Fix the help text to restart GitLab in case of issues with Sidekiq. The correct command is copied from https://docs.gitlab.com/omnibus/maintenance/.

Are there points in the code the reviewer needs to double check?

I only corrected the syntax of the shell command, not the semantics. It might make sense to only restart Sidekiq (the purpose of restarting is not outlined in the help text). Furthermore, a warning might be nice – I assume that restarting GitLab corrupted an active GitHub Mirroring and lead to some SQL inconsistency issues.

Why was this MR needed?

The current help text contains an invalid shell command.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)