docs(getting-started): switch @types/hammerjs to dev dependency on instructions

Suggest developers to install @types/hammerjs as a dev dependency instead of a plain dependency.

Closes #1889

Author: Drumarcco

GETTING_STARTED.md

@@ -47,7 +47,8 @@ Add HammerJS to your application via [npm](https://www.npmjs.com/package/hammerj
 directly from your app.
 
 ```bash
-npm install --save hammerjs @types/hammerjs
+npm install --save hammerjs 
+npm install --sade-dev @types/hammerjs
 ```
 
 Import HammerJS on your app's module.