Correct npm instructions in Intermediate Tutorial

This makes the `npm install` command match the one in the Quick Start.

Author: roryokane

docs/tutorials/intermediate-tutorial.md

@@ -48,7 +48,7 @@ In the Basic Tutorial, we just linked to Redux Toolkit as an individual script t
 
 ```bash
 # If you're using NPM:
-npm install @reduxjs/toolkit
+npm install --save @reduxjs/toolkit
 
 # Or for Yarn:
 yarn add @reduxjs/toolkit