Fix mkdocs (#6003)

* Fixed mkdocs config.

* Updated README.

Author: miguelalonsojr

docs/Readme.md

@@ -44,7 +44,7 @@ See our [ML-Agents Overview](ML-Agents-Overview.md) page for detailed
 descriptions of all these features. Or go straight to our [web docs](https://unity-technologies.github.io/ml-agents/).
 ## Releases & Documentation
 
-**Our latest, stable release is `Release 20`. Click
+**Our latest, stable release is `Release 21`. Click
 [here](Getting-Started.md)
 to get started with the latest release of ML-Agents.**
 

mkdocs.yml

@@ -38,8 +38,8 @@ theme:
   logo: images/unity-logo.png
   favicon: images/unity-logo-black.png
   palette:
-    primary: blue
-    accent: light blue
+    - primary: blue
+    - accent: light blue
     # Palette toggle for light mode
     - scheme: default
       toggle: