desktop-shell: Use 'surface-local' to correct grammar

Correct the pluralization by simplifying with 'surface-local'.

See https://lists.freedesktop.org/archives/wayland-devel/2016-April/028249.html.

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

desktop-shell/shell.c

@@ -1869,7 +1869,7 @@ static const struct weston_pointer_grab_interface resize_grab_interface = {
 
 /*
  * Returns the bounding box of a surface and all its sub-surfaces,
- * in the surface coordinates system. */
+ * in surface-local coordinates. */
 static void
 surface_subsurfaces_boundingbox(struct weston_surface *surface, int32_t *x,
 				int32_t *y, int32_t *w, int32_t *h) {