Update docs URL

Author: jgu2

README.md

@@ -14,7 +14,7 @@ Engage is a free, open-access energy system planning tool that allows multiple u
 
 Please refer to the documentation to setup development environment,
 
-https://nrel.github.io/engage/index.html
+https://nrel.github.io/engage
 
 and the developer guide to contribute to Enage project.