xdg-shell: Correct grammar

Adjust minor grammar issues, for clarity.

This patch cherry-picks some relevant changes from an earlier series,
patches 3 to 5. See:
https://lists.freedesktop.org/archives/wayland-devel/2016-April/028078.html



Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>