update readme

Author: andreivictor

README.md

@@ -13,7 +13,7 @@ There are 5 predefined custom classes in CSS: `.tooltip-primary`, `.tooltip-succ
 To install latest version via `npm` run:
 
 ```
-npm install bootstrap-tooltip-custom-class
+$ npm install bootstrap-tooltip-custom-class
 ```
 
 ## Setup