doc/conf: fix sphinx language setting

sphinx-build 5.1.1 and above throws a warning which is treated as an
error while building.

Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).

diff --git a/doc/userguide/conf.py b/doc/userguide/conf.py
index 1474dd87afb83e108ecb777de4da1de3185db250..47fec8ec6c24e83a9960da9f29779d3ff7ed0ae4 100644 (file)
--- a/doc/userguide/conf.py
+++ b/doc/userguide/conf.py
@@ -80,7 +80,7 @@ release = version
 #
 # 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'
 
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used: