added sourcesort url to guide

Author: robcotter

_articles/how-to-contribute.md

@@ -224,7 +224,7 @@ You can also use one of the following resources to help you discover and contrib
 * [Up For Grabs](https://up-for-grabs.net/)
 * [Contributor-ninja](https://contributor.ninja)
 * [First Contributions](https://firstcontributions.github.io)
-* [SourceSort](https://sourcesort.com/)
+* [SourceSort](https://www.sourcesort.com/)
 
 ### A checklist before you contribute