Fixed broken link to discuss.react.org (#2107)

* Replaced broken discuss.reactjs.org link

On the how-to-contribute page, there is a broken link under the https://reactjs.org/docs/how-to-contribute.html#how-to-get-in-touch section. As outlined in https://github.com/reactjs/reactjs.org/issues/2080 `discuss.reactjs.org` isn't reachable.

I edited the text to display `Discussion Forums` which links to https://reactjs.org/community/support.html#popular-discussion-forums (as I was unable to find an official reactjs discussion forum).

* fixed text case

changed `Discussion Forums` to `Discussion forums`

content/docs/how-to-contribute.md

@@ -50,7 +50,7 @@ Facebook has a [bounty program](https://www.facebook.com/whitehat/) for the safe
 ### How to Get in Touch {#how-to-get-in-touch}
 
 * IRC: [#reactjs on freenode](https://webchat.freenode.net/?channels=reactjs)
-* Discussion forum: [discuss.reactjs.org](https://discuss.reactjs.org/)
+* [Discussion forums](https://reactjs.org/community/support.html#popular-discussion-forums)
 
 There is also [an active community of React users on the Discord chat platform](https://www.reactiflux.com/) in case you need help with React.