docs: add a code sample for computed env variables declaration (#3046) [ci skip]

Fixes #2864

Author: NataliaTepluhina

docs/guide/mode-and-env.md

@@ -86,7 +86,15 @@ In addition to `VUE_APP_*` variables, there are also two special variables that
 All resolved env variables will be available inside `public/index.html` as discussed in [HTML - Interpolation](./html-and-static-assets.md#interpolation).
 
 ::: tip
-You can have computed env vars in your `vue.config.js` file. They still need to be prefixed with `VUE_APP_`. This is useful for version info `process.env.VUE_APP_VERSION = require('./package.json').version`
+You can have computed env vars in your `vue.config.js` file. They still need to be prefixed with `VUE_APP_`. This is useful for version info
+
+```js
+process.env.VUE_APP_VERSION = require('./package.json').version
+
+module.exports = {
+  // config
+}
+```
 :::
 
 ## Local Only Variables