Update example config

According to the docs: https://github.com/postcss/postcss-loader#path

One cannot change the filename, only the path.

Author: Nyholm

frontend/encore/postcss.rst

@@ -48,7 +48,8 @@ You can also pass options to the `postcss-loader`_ by passing a callback:
         // ...
     +     .enablePostCssLoader((options) => {
     +         options.config = {
-    +             path: 'app/config/postcss.config.js'
+    +             // the directory where the postcss.config.js file is stored
+    +             path: 'path/to/config'
     +         };
     +     })
     ;