protocol: fix grammar

a icon -> an icon

Signed-off-by: Abdur Rehman <arehmanq199@gmail.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>

protocol/ivi-hmi-controller.xml

@@ -79,7 +79,7 @@
         workspace-id=0
 		: id of workspace to add a launcher
         icon-id=4001
-		: ivi id of ivi_surface to draw a icon
+		: ivi id of ivi_surface to draw an icon
         icon=/home/user/review/build-ivi-shell/data/icon_ivi_flower.png
 		: path to icon image
         path=/home/user/review/build-ivi-shell/weston-dnd