fix adafruit#1833, replace expired discord link

mchua · web-flow · commit 493fe971b579 · 2019-05-07T22:26:00.000-04:00
The footer already links directly to https://adafru.it/discord, which seems like a more permanent location. I've just replaced the expired link at the top with the same thing.
diff --git a/README.rst b/README.rst
@@ -63,7 +63,7 @@ Contributors who follow the `Code of
 Conduct <https://github.com/adafruit/circuitpython/blob/master/CODE_OF_CONDUCT.md>`__
 are welcome to submit pull requests and they will be promptly reviewed
 by project admins. Please join the
-`Discord <https://discord.gg/nBQh6qu>`__ too.
+`Discord <http://adafru.it/discord>`__ too.
 
 Branding
 ------------
