fix(docs): prevent a sphinx warning

Some newer versions of sphinx (tried on v5.3) will warn about language
being None which will fail the build. Change it to the default (en) to
prevent this.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Ie0570481f42aeb293e885ca936e0765f6cb299a8
diff --git a/docs/conf.py b/docs/conf.py
index 41947d9..371632a 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -36,7 +36,7 @@
 #
 # 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.