Add ERC721 import in tutorial

docs/modules/ROOT/pages/erc721.adoc

@@ -16,10 +16,11 @@ Here's what a contract for tokenized items might look like:
 // SPDX-License-Identifier: MIT
 pragma solidity ^0.8.0;
 
+import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
 import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";
 import "@openzeppelin/contracts/utils/Counters.sol";
 
-contract GameItem is ERC721URIStorage {
+contract GameItem is ERC721, ERC721URIStorage {
     using Counters for Counters.Counter;
     Counters.Counter private _tokenIds;