fix(docs): set language to "en" in sphinx config

Sphinx complains about the lack of a language in the configuration. Our
documentation is in English, so it makes sense to have that as an
explicit setting the config - TF-A already does this.

Change-Id: Iacadc870116e189080febbd986eead7dfd9a964d
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/docs/conf.py b/docs/conf.py
index 58f5e72..49c3343 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.