docs: Set language to english. Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>

diff --git a/docs/conf.py b/docs/conf.py
index 35a232f..6db5516 100644
--- a/docs/conf.py
+++ b/docs/conf.py

@@ -105,7 +105,7 @@
 #
 # 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 = 'en'
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.