protocol: Describe wl_registry as singleton

Other singleton objects in the protocol are described as such.

Add a singleton adjective to the wl_registry description, making it
similar to other descriptions.

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