Correct npm instructions in Intermediate Tutorial (#340)

As of npm v5, `--save` is no longer necessary with `npm install`. Removing that flag makes the `npm install` command match the one given in the Intermediate Tutorial.

Author: roryokane

docs/introduction/quick-start.md

@@ -39,7 +39,7 @@ Redux Toolkit is available as a package on NPM for use with a module bundler or
 
 ```bash
 # NPM
-npm install --save @reduxjs/toolkit
+npm install @reduxjs/toolkit
 
 # Yarn
 yarn add @reduxjs/toolkit