Hint to run finalize command in a compatible shell

What does this MR do and why?

We had a customer that attempted to run the background migration finalize command in tcsh and it failed to run. We worked out that the commands should be run in bash or sh to actually run.

This MR adjusts the text so that it hints that the finalize commands should be run in bash or sh.

Internal ticket: https://gitlab.zendesk.com/agent/tickets/285961

Tests

# Customer Provided. Using `tcsh`
$ sudo gitlab-rake gitlab:background_migrations:finalize[ProjectNamespaces::BackfillProjectNamespaces,projects,id,'[null\,"up"]']
sudo: No match.

# Using `zsh`
$ sudo gitlab-rake gitlab:background_migrations:finalize[ProjectNamespaces::BackfillProjectNamespaces,projects,id,'[null\,"up"]']
zsh: no matches found: gitlab:background_migrations:finalize[ProjectNamespaces::BackfillProjectNamespaces,projects,id,[null\,"up"]]

# Using `bash`
$ sudo gitlab-rake gitlab:background_migrations:finalize[ProjectNamespaces::BackfillProjectNamespaces,projects,id,'[null\,"up"]']
Done.

# Using `sh`
$ sudo gitlab-rake gitlab:background_migrations:finalize[ProjectNamespaces::BackfillProjectNamespaces,projects,id,'[null\,"up"]']
Done.

MR acceptance checklist

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