Add fix #2216 in CHANGELOG [ci skip]

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 v 7.14.0 (unreleased)
+  - Fix coloring of diffs on MR Discussion-tab (Gert Goet)
   - Fix "Network" and "Graphs" pages for branches with encoded slashes (Stan Hu)
   - Fix errors deleting and creating branches with encoded slashes (Stan Hu)
   - Fix multi-line syntax highlighting (Stan Hu)