Fix issue_closing_regex in gitlab.yml.example

It turns out that regex escapes and YAML quoted-scalar escapes don't play
nicely together.

Before (with `issue_closing_pattern` uncommented):
```
smash@winter:config example-config-fix *$ irb -ryaml
2.0.0-p247 :001 > YAML.load_file('gitlab.yml.example')
Psych::SyntaxError: (gitlab.yml.example): found unknown escape character while parsing a quoted scalar at line 52 column 28
```

After:
```
smash@winter:config example-config-fix *$ irb -ryaml
2.0.0-p247 :001 > YAML.load_file('gitlab.yml.example')
 => {"production"=>{"gitlab"=>{"host"=>"localhost", "port"=>80, "https"=>false, "email_from"=>"gitlab@localhost", "support_email"=>"support@localhost", "default_projects_limit"=>10, "issue_closing_pattern"=>"^([Cc]loses|[Ff]ixes) +#\\d+", "default_projects_features"=>{"issues"=>true, "merge_requests"=>true, ...
```