Fix tox-docs failing sphinx

A new version of sphinx was released May 29 2022
which requires a language setting in config otherwise
a warning (treated as error) causes the sphinx operation
to fail.

Updated the sphinx config file to correct the issue.

The sphinx behavioural change is mentioned here:
sphinx-doc/sphinx#10062
sphinx-doc/sphinx#10474

Partial-Bug: #1976377
Signed-off-by: Al Bailey <al.bailey@windriver.com>
Change-Id: I5fea877955c0d9b875c31f0b64bc8f3bced5c208

doc/source/conf.py

@@ -62,7 +62,7 @@
 #
 # 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.