Update callback.md
Update callback.md

callback isn't a pointer. Member access operator is incorrect.

gagern/nan,nodejs/nan,eljefedelrodeodeljefe/nan,gagern/nan,eljefedelrodeodeljefe/nan,nodejs/nan,nodejs/nan,eljefedelrodeodeljefe/nan,addaleax/nan,gagern/nan,eljefedelrodeodeljefe/nan,nodejs/nan,addaleax/nan,addaleax/nan,addaleax/nan