docs: Set language to English.

Fixes https://github.com/google/gf180mcu-pdk/issues/21.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>

docs/conf.py

diff --git a/docs/conf.py b/docs/conf.py
index 1edfdbd..d38270a 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -104,7 +104,7 @@
 #
 # 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.