changed variable name (#1594)

changed variable name, it sounds like my directory

Author: mod7ex

src/guide/reusability/custom-directives.md

@@ -108,7 +108,7 @@ Custom directives should only be used when the desired functionality can only be
 A directive definition object can provide several hook functions (all optional):
 
 ```js
-const myDir = {
+const myDirective = {
   // called before bound element's attributes
   // or event listeners are applied
   created(el, binding, vnode, prevVnode) {