Fix typo for comm. MRs policy filename

When creating or changing a new triage policy or triage package:

• Consider adding documentation to the handbook pages for Triage Operations: https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/source/handbook/engineering/quality/triage-operations/index.html.md
• Consider announcing to the appropriate Slack channels or week-in-review

Summary

Fixes:

 $ gitlab-triage -r ./plugins/all --debug -f $TRIAGE_POLICY_FILE --token $GITLAB_BOT_AUTOMATED_TRIAGE_TOKEN --source $TRIAGE_SOURCE_TYPE --source-id $TRIAGE_SOURCE_PATH $EXTRA_FLAGS
 /usr/local/lib/ruby/2.6.0/psych.rb:577:in `initialize': No such file or directory @ rb_sysopen - policies/stages/hygiene/add-milestone-to-community-contributions.yml (Errno::ENOENT)
 	from /usr/local/lib/ruby/2.6.0/psych.rb:577:in `open'
 	from /usr/local/lib/ruby/2.6.0/psych.rb:577:in `load_file'
 	from /usr/local/bundle/gems/gitlab-triage-1.7.1/bin/gitlab-triage:11:in `block in <top (required)>'
 	from /usr/local/lib/ruby/2.6.0/set.rb:338:in `each_key'
 	from /usr/local/lib/ruby/2.6.0/set.rb:338:in `each'
 	from /usr/local/bundle/gems/gitlab-triage-1.7.1/bin/gitlab-triage:10:in `<top (required)>'
 	from /usr/local/bundle/bin/gitlab-triage:23:in `load'
 	from /usr/local/bundle/bin/gitlab-triage:23:in `<main>'

References

Mentions: !458 (merged) Closes #451 (closed)