Valid issue closing pattern docs example

What does this MR do?

The suggested addition for gitlab_issue_closing_pattern to gitlab.rb produces a different regular expression than expected because it is encoded as a Ruby string, so escapes produce different results. This change writes the regular expression in Ruby's Regexp syntax and then converts it to the source string to preserve the syntax of the regular expression with the following YAML example.

Related issues

After a search for gitlab_issue_closing_pattern in the issue tracker, I believe this is the solution to #218641 (closed).

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