Convert unexplained variables to arguments in an emits example

vuejs/docs@2400973#diff-0ddfe84e020022e6d0eef551720a870386146a8703934cd0cc00a952e651e402

Author: naokie

src/guide/component-custom-events.md

@@ -57,7 +57,7 @@ app.component('custom-form', {
     }
   },
   methods: {
-    submitForm() {
+    submitForm(email, password) {
       this.$emit('submit', { email, password })
     }
   }