Removed unused nextId variable (#6470)

Author: TymeeK

src/content/learn/updating-arrays-in-state.md

@@ -409,7 +409,6 @@ For example:
 ```js
 import { useState } from 'react';
 
-let nextId = 3;
 const initialList = [
   { id: 0, title: 'Big Bellies' },
   { id: 1, title: 'Lunar Landscape' },