message: Remove trailing '/' from Mailing list link in README file
diff: @@ -128,7 +128,7 @@ Installation & Configuration
  Resources
  -------------
  -* [Mailing list](https://lists.apache.org/list.html?dev@superset.apache.org/)
  +* [Mailing list](https://lists.apache.org/list.html?dev@superset.apache.org)
  * [Gitter (live chat) Channel](https://gitter.im/airbnb/superset)
  * [Docker image](https://hub.docker.com/r/amancevice/superset/) (community contributed)
  * [Slides from Strata (March 2016)](https://drive.google.com/open?id=0B5PVE0gzO81oOVJkdF9aNkJMSmM)
  