Removing Vue.config.debug usage from documentation (#871)

* Removing Vue.config.debug usage from documentation

It has been removed; https://vuejs.org/v2/guide/migration.html#Vue-config-debug-removed

* Update env.md

Author: adam-lynch

docs/env.md

@@ -42,10 +42,10 @@ So, the environment variables are:
 
 As we can see, `test.env` inherits the `dev.env` and the `dev.env` inherits the `prod.env`.
 
-### Usage
+### Usage		
 
-It is simple to use the environment variables in your code. For example:
+It is simple to use the environment variables in your code. For example:		
 
-```js
-Vue.config.debug = process.env.DEBUG_MODE
-```
+```js		
+Vue.config.productionTip = process.env.NODE_ENV === 'production'		
+```