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
1 file changed