Update custom-directives.md (vuejs#2885)

Removing `prevVnode` params from hooks where they aren't available.

Author: thiagosaife

src/guide/reusability/custom-directives.md

@@ -111,23 +111,23 @@ A directive definition object can provide several hook functions (all optional):
 const myDirective = {
   // called before bound element's attributes
   // or event listeners are applied
-  created(el, binding, vnode, prevVnode) {
+  created(el, binding, vnode) {
     // see below for details on arguments
   },
   // called right before the element is inserted into the DOM.
-  beforeMount(el, binding, vnode, prevVnode) {},
+  beforeMount(el, binding, vnode) {},
   // called when the bound element's parent component
   // and all its children are mounted.
-  mounted(el, binding, vnode, prevVnode) {},
+  mounted(el, binding, vnode) {},
   // called before the parent component is updated
   beforeUpdate(el, binding, vnode, prevVnode) {},
   // called after the parent component and
   // all of its children have updated
   updated(el, binding, vnode, prevVnode) {},
   // called before the parent component is unmounted
-  beforeUnmount(el, binding, vnode, prevVnode) {},
+  beforeUnmount(el, binding, vnode) {},
   // called when the parent component is unmounted
-  unmounted(el, binding, vnode, prevVnode) {}
+  unmounted(el, binding, vnode) {}
 }
 ```