Fix gitter url

The https scheme is needed as check-links.py checks for 200. Pdfx
prepends http:// to links without a scheme which results in a permanent
redired (308) to 'https://gitter.im/matplotlib/matplotlib' and hence
causes check-links.py to fail.

Author: StefRe

cheatsheets.tex

@@ -328,7 +328,7 @@
     \href{https://stackoverflow.com/questions/tagged/matplotlib}
          {\faIcon{stack-overflow}\,stackoverflow.com/questions/tagged/matplotlib}\\
     \href{https://gitter.im/matplotlib/matplotlib}
-         {\faIcon{gitter}\,gitter.im/matplotlib}\\
+         {\faIcon{gitter}\,{https://gitter.im/matplotlib/matplotlib}}\\
     \href{https://twitter.com/matplotlib}
          {\faIcon{twitter}\,twitter.com/matplotlib}\\
     \href{https://mail.python.org/mailman/listinfo/matplotlib-users}