fix sphinx warnings

docs/conf.py

@@ -259,6 +259,6 @@ extensions = [
 ]
 
 extlinks = {
-    'issue': ('https://github.com/borgbackup/borg/issues/%s', '#'),
+    'issue': ('https://github.com/borgbackup/borg/issues/%s', '#%s'),
     'targz_url': ('https://pypi.python.org/packages/source/b/borgbackup/%%s-%s.tar.gz' % version, None),
 }