Use "username" instead of "user name"

Author: sebastianbergmann

.github/CONTRIBUTING.md

@@ -56,7 +56,7 @@ Please use the most specific issue tracker to search for existing tickets and to
 5. Ensure the code complies with our coding guidelines (see below).
 6. Send that pull request!
 
-Please make sure you have [set up your user name and email address](https://git-scm.com/book/en/v2/Getting-Started-First-Time-Git-Setup) for use with Git. Strings such as `silly nick name <root@localhost>` look really stupid in the commit history of a project.
+Please make sure you have [set up your username and email address](https://git-scm.com/book/en/v2/Getting-Started-First-Time-Git-Setup) for use with Git. Strings such as `silly nick name <root@localhost>` look really stupid in the commit history of a project.
 
 We encourage you to [sign your Git commits with your GPG key](https://docs.github.com/en/github/authenticating-to-github/signing-commits).