Fix typo (#234)

* Fix typo

It should be `stubs` instead of `stub`.

* Fix typo

Author: jaylinski

docs/en/api/mount.md

@@ -122,8 +122,8 @@ import Faz from './Faz.vue'
 describe('Foo', () => {
   it('renders a div', () => {
     const wrapper = mount(Foo, {
-      stub: {
-        Bar: '<div class="stubbed />',
+      stubs: {
+        Bar: '<div class="stubbed" />',
         BarFoo: true,
         FooBar: Faz
       }