chore: switch javascript code blocks to js

vuejs/docs@5753b78#diff-7d4d6d18bce98fb202b53d0597ea1e4a7ba7f467b9db5a86e4c25d8d59fc7fe0

Author: naokie

src/guide/single-file-component.md

@@ -90,7 +90,7 @@ touch rollup.config.js
 
 Once the file is created we will need to open it with our editor of choice and add the following code.
 
-```javascript
+```js
 // import our third party plugins
 import commonjs from 'rollup-plugin-commonjs'
 import VuePlugin from 'rollup-plugin-vue'
@@ -144,7 +144,7 @@ Here we are specifying:
 To also build `umd` and `cjs` modules we can simply add a few lines of configuration to our `rollup.config.js` and `package.json`
 
 ##### rollup.config.js
-```javascript
+```js
 output: [
   ...
    {