Update unit-testing.md (#430)

Fixed reference to function argument in helper function

Author: gaje

src/guide/unit-testing.md

@@ -93,7 +93,7 @@ import MyComponent from './MyComponent.vue'
 
 // helper function that mounts and returns the rendered text
 function getRenderedText (Component, propsData) {
-  const Ctor = Vue.extend(MyComponent)
+  const Ctor = Vue.extend(Component)
   const vm = new Ctor({ propsData }).$mount()
   return vm.$el.textContent
 }