Fix gitter link

Author: dstansby

cheatsheets.tex

@@ -327,7 +327,7 @@
          {\faIcon{discourse}\,discourse.matplotlib.org}\\
     \href{https://stackoverflow.com/questions/tagged/matplotlib}
          {\faIcon{stack-overflow}\,stackoverflow.com/questions/tagged/matplotlib}\\
-    \href{https://gitter.im/matplotlib/matplotlib}
+    \href{https://app.gitter.im/#/room/#matplotlib_matplotlib:gitter.im}
          {\faIcon{gitter}\,{https://gitter.im/matplotlib/matplotlib}}\\
     \href{https://twitter.com/matplotlib}
          {\faIcon{twitter}\,twitter.com/matplotlib}\\