Ensure issue closing pattern example is valid

The suggested addition 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.

doc/administration/issue_closing_pattern.md

@@ -12,7 +12,7 @@ This is the administration documentation. There is a separate [user documentatio
 on issue closing pattern.
 
 When a commit or merge request resolves one or more issues, it is possible to
-automatically have these issues closed when the commit or merge request lands
+automatically close these issues when the commit or merge request lands
 in the project's default branch.
 
 ## Change the issue closing pattern
@@ -35,7 +35,7 @@ Because Rubular doesn't understand `%{issue_ref}`, you can replace this by
    expression of your liking:
 
    ```ruby
-   gitlab_rails['gitlab_issue_closing_pattern'] = "\b((?:[Cc]los(?:e[sd]?|ing)|\b[Ff]ix(?:e[sd]|ing)?|\b[Rr]esolv(?:e[sd]?|ing)|\b[Ii]mplement(?:s|ed|ing)?)(:?) +(?:(?:issues? +)?%{issue_ref}(?:(?: *,? +and +| *,? *)?)|([A-Z][A-Z0-9_]+-\d+))+)"
+   gitlab_rails['gitlab_issue_closing_pattern'] = /\b((?:[Cc]los(?:e[sd]?|ing)|\b[Ff]ix(?:e[sd]|ing)?|\b[Rr]esolv(?:e[sd]?|ing)|\b[Ii]mplement(?:s|ed|ing)?)(:?) +(?:(?:issues? +)?%{issue_ref}(?:(?: *,? +and +| *,? *)?)|([A-Z][A-Z0-9_]+-\d+))+)/.source
    ```
 
 1. [Reconfigure](restart_gitlab.md#omnibus-gitlab-reconfigure) GitLab for the changes to take effect.