reference: link to our own cheat sheet

From the reference overview we were linking to two cheat sheets:

* https://training.github.com/: By GitHub, but doesn't look really well
  maintained.
* https://ndpsoftware.com/git-cheatsheet.html: Which looks fancy, but is
  behind a self-signed SSL certificate.

Since git#2049 we have our own cheat
sheet, link to that one instead.

Author: To1ne

layouts/partials/ref/index.html

@@ -3,10 +3,8 @@
   <h1>Reference</h1>
   <div class='callout quickref'>
     <p>
-      Quick reference guides:
-      <a href="https://github.github.com/training-kit/">GitHub Cheat Sheet</a>
-      |
-      <a href="https://ndpsoftware.com/git-cheatsheet.html">Visual Git Cheat Sheet</a>
+      Quick reference guide:
+      <a href="../cheat-sheet">Cheat Sheet</a>
     </p>
   </div>
   <div class="callout all-commands">