Fix standalone vue.js cdn url (#1146)

The other one results in `vue.common.js:245 Uncaught ReferenceError: process is not defined` while loading

Author: gabor-farkas

src/v2/guide/index.md

@@ -17,7 +17,7 @@ If you are an experienced frontend developer and want to know how Vue compares t
 The easiest way to try out Vue.js is using the [JSFiddle Hello World example](https://jsfiddle.net/chrisvfritz/50wL7mdz/). Feel free to open it in another tab and follow along as we go through some basic examples. Or, you can <a href="https://gist.githubusercontent.com/chrisvfritz/7f8d7d63000b48493c336e48b3db3e52/raw/ed60c4e5d5c6fec48b0921edaed0cb60be30e87c/index.html" target="_blank" download="index.html">create an <code>index.html</code> file</a> and include Vue with:
 
 ``` html
-<script src="https://cdn.jsdelivr.net/npm/vue"></script>
+<script src="https://unpkg.com/vue"></script>
 ```
 
 The [Installation](installation.html) page provides more options of installing Vue. Note: We **do not** recommend that beginners start with `vue-cli`, especially if you are not yet familiar with Node.js-based build tools.