pointer: fix documentation

Previously it uses event.Op{}, but such struct don't
exists anymore. Instead, it have a function with the
same name.

Signed-off-by: inkeliz <inkeliz@inkeliz.com>

io/pointer/doc.go

@@ -37,11 +37,11 @@ For example:
 	var h1, h2 *Handler
 
 	area := clip.Rect(...).Push(ops)
-	event.Op{Tag: h1}.Add(Ops)
+	event.Op(Ops, h1)
 	area.Pop()
 
 	area := clip.Rect(...).Push(ops)
-	event.Op{Tag: h2}.Add(ops)
+	event.Op(Ops, h2)
 	area.Pop()
 
 implies a tree of two inner nodes, each with one pointer handler attached.