removes invalid character in comments (#2571)

contracts/token/ERC721/ERC721.sol

@@ -223,7 +223,7 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
      * @dev Safely mints `tokenId` and transfers it to `to`.
      *
      * Requirements:
-     d*
+     *
      * - `tokenId` must not exist.
      * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
      *