Simple NFT

MyNFT2.sol

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/ERC721.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/utils/Strings.sol";

contract MyNFT2 is ERC721 {

    address owner;
    uint public nextTokenId = 0;
    string uri = "https://crypto.salamat.tokyo/sandbox/nft1/nft_metadata3.json?id=";

    constructor() ERC721("Stop NFT MoneyMonger NFT", "SNFT"){
        owner = msg.sender;
    }

    function mint() public {
        uint256 tokenId = nextTokenId;
        nextTokenId = nextTokenId + 1;
        super._safeMint(msg.sender, tokenId);
    }

    function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
        return string(abi.encodePacked(uri, Strings.toString(tokenId)));
    }

    //万一画像の置き場所を変えたくなったとき用
    function changeUri(string memory _uri) public {
        require(owner == msg.sender);
        uri = _uri;
    }
}