fix rocket.chat URL (#1175)

Thanks @evaryont

.github/CONTRIBUTING.md

@@ -1,6 +1,6 @@
 ## Contributing
 
-Please join us on [chat](https://chat.awesome-selfhosted.com) to clarify any questions.
+Please join us on [chat](https://chat.awesh.unknownplus.com/channel/awesome-selfhosted) to clarify any questions.
 
  * The **syntax** for links is ``[Name](http://homepage/) - Short description. ([Demo](http://url.to/demo), [Source Code](http://url.of/source/code)) `License` `Language` ``. All projects should have their **license** and main server-side **language**/platform/requirement listed, and a description at most 250 characters long.
  * To **add an entry**: clone/[download](https://github.com/Kickball/awesome-selfhosted/archive/master.zip) and enter the repository, run `make add` and follow the instructions; or [edit the README.md file](https://github.com/Kickball/awesome-selfhosted/edit/master/README.md) directly. See [Editing files in another user's repository](https://help.github.com/articles/editing-files-in-another-user-s-repository/), [Creating Pull Requests](https://help.github.com/articles/creating-a-pull-request/), [Using Pull Requests](https://help.github.com/articles/using-pull-requests/) for help on sending your patch.