drm/docs: Fix usage stats typos

Fix a couple missing ':'s.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230524155956.382440-2-robdclark@gmail.com