Documentation for the prepare_lock_surface event description, is incorrect

Documentation for the prepare_lock_surface event description is
incorrect. The summary says "Tell the client..." however the full-text
description says "tell the shell..."

Signed-off-by: Chris Michael <cp.michael@samsung.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

[This time include the actual changes.]