Fix config error

Newer Sphinx doesn't accept None, so leaving default of 'en'.

source/conf.py

@@ -92,7 +92,7 @@
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+# language = None
 
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used: