doc: fix libinput_event_destroy parameter

Fix libinput_event_destroy call by passing the correct parameter (event) in the
example code.

Signed-off-by: Diego Rondini <diego.rondini@kynetics.com>

doc/api/mainpage.dox

@@ -96,7 +96,7 @@ int main(void) {
 
 		// handle the event here
 
-		libinput_event_destroy(li);
+		libinput_event_destroy(event);
 		libinput_dispatch(li);
 	}