Fix variable reference in $mount sample code. (vuejs#501)

Author: patrick-steele

src/api/index.md

@@ -1176,7 +1176,7 @@ All lifecycle hooks automatically have their `this` context bound to the instanc
 
   // or, render off-document and append afterwards:
   var component = new MyComponent().$mount()
-  document.getElementById('app').appendChild(vm.$el)
+  document.getElementById('app').appendChild(component.$el)
   ```
 
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