Update URL in the About dialog box

Better reflects where updates are to be expected.

Author: barijaona

Resources/Credits.html

@@ -7,5 +7,5 @@
 </style>
 <p>By developers, for developers.</p>
 <p style="margin: 0px; padding: 0px">
-  <a href="https://rowanj.github.io/gitx/">https://rowanj.github.io/gitx/</a>
-</p>
+  <a href="https://github.com/gitx/gitx">https://github.com/gitx/gitx</a>
+</p>