app: update EndPaint in test and documentation

Change-Id: I7c9e6edc2d50dca3c2d15426370d218d0c4dfe09
Reviewed-on: https://go-review.googlesource.com/13021
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

app/doc.go

@@ -57,7 +57,7 @@ goroutine as other code that calls OpenGL.
 					// ...
 				case paint.Event:
 					log.Print("Call OpenGL here.")
-					a.EndPaint()
+					a.EndPaint(e)
 				}
 			}
 		})

app/internal/testapp/testapp.go

@@ -59,7 +59,7 @@ func main() {
 					comm.Send("paint")
 					sendPainting = false
 				}
-				a.EndPaint()
+				a.EndPaint(e)
 			case touch.Event:
 				comm.Send("touch", e.Type, e.Loc.X.Px(c.PixelsPerPt), e.Loc.Y.Px(c.PixelsPerPt))
 			}