protocol: Use singleton global consistently

The protocol describes wl_shm as a "global singleton" rather than
"singleton global," which is the order used throughout other protocol
object descriptions.

Re-order the terms for consistency.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>