Fix docs folder

When attempting to edit a file on ReadTheDocs, it simply appends
`github_folder` to the file name, so we need the trailing path
separator.

docs/custom_conf.py

@@ -72,7 +72,7 @@
 
     # Change to the folder that contains the documentation
     # (usually "/" or "/docs/")
-    'github_folder': '/docs',
+    'github_folder': '/docs/',
 
     # Change to an empty value if your GitHub repo doesn't have issues enabled.
     # This will disable the feedback button and the issue link in the footer.