Update postcss.md (vuejs#1034)

2hu12 · kazupon · commit 069bc3142a3d · 2017-11-03T00:34:03.000+09:00
Just add quotes for PostCSS option value to make it valid.
diff --git a/docs/en/features/postcss.md b/docs/en/features/postcss.md
@@ -62,7 +62,7 @@ In addition to providing an Array of plugins, the `postcss` option also accepts:
   postcss: {
     plugins: [...], // list of plugins
     options: {
-      parser: sugarss // use sugarss parser
+      parser: 'sugarss' // use sugarss parser
     }
   }
   ```

Original file line number	Diff line number	Diff line change
@@ -62,7 +62,7 @@ In addition to providing an Array of plugins, the `postcss` option also accepts:
`62`	`62`	`postcss: {`
`63`	`63`	`plugins: [...], // list of plugins`
`64`	`64`	`options: {`
`65`		`- parser: sugarss // use sugarss parser`
	`65`	`+ parser: 'sugarss' // use sugarss parser`
`66`	`66`	`}`
`67`	`67`	`}`
`68`	`68`	```