Fixed a minor typo in CONTRIBUTING.md

Author: smgarmin

CONTRIBUTING.md

@@ -18,7 +18,7 @@ For example:
 ```
 git clone https://github.com/<github_username>/firstcontributions.github.io.git
 ```
-where `<github_sername>` is your GitHub username.
+where `<github_username>` is your GitHub username.
 Here you're copying the contents of your firstcontributions.github.io repository on GitHub to your computer.
 
 ## Running The Project