Update installation.md

@2 prefix has to be specified, otherwise v3 gets installed

Author: tno2007

src/v2/guide/installation.md

@@ -71,7 +71,7 @@ NPM is the recommended installation method when building large scale application
 
 ``` bash
 # latest stable
-$ npm install vue
+$ npm install vue@2
 ```
 
 ## CLI