[docs] Fix a typo in a link in the package docs (#4161)

com.unity.ml-agents/Documentation~/com.unity.ml-agents.md

 ## Helpful links
 
 If you are new to the Unity ML-Agents package, or have a question after reading
-the documentation, you can checkout our [GitHUb Repository], which also includes
+the documentation, you can checkout our [GitHub Repository], which also includes
 a number of ways to [connect with us] including our [ML-Agents Forum].
 
 [unity ML-Agents Toolkit]: https://github.com/Unity-Technologies/ml-agents