docs: update required mako version

The requirement was bumped a while back, but we forgot to update the
docs.

Fixes: ed871af9 ("configure.ac: raise Mako required version to
0.8.0")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit e39b916d)