Simplify ERC721 example (#3115)

docs/modules/ROOT/pages/erc721.adoc

@@ -16,7 +16,6 @@ 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";