Fixed link to contributing guidelines.

Absolute paths in links end up being rooted at github.com.
The contributing guidelines link is broken unless we use the full URL.
Also, remove superfluous "monospace formatting" for the link.

.github/pull_request_template.md

@@ -6,5 +6,5 @@ If this pull request addresses an issue on GitHub, make sure to reference that
 issue using one of the
 [supported keywords](https://docs.github.com/en/github/managing-your-work-on-github/linking-a-pull-request-to-an-issue).
 
-Before creating a pull request, make sure to comply with
-[`Contributing Guidelines`](/CONTRIBUTING.md).
+Before creating a pull request, make sure to comply with the
+[Contributing Guidelines](https://github.com/nginx/nginx/blob/master/CONTRIBUTING.md).