Fix whitespace in IERC721Receiver (#2359)

Slight nit on spacing to reduce # of lines // conform to other OZ function syntax

contracts/token/ERC721/IERC721Receiver.sol

@@ -17,6 +17,5 @@ interface IERC721Receiver {
      *
      * The selector can be obtained in Solidity with `IERC721.onERC721Received.selector`.
      */
-    function onERC721Received(address operator, address from, uint256 tokenId, bytes calldata data)
+    function onERC721Received(address operator, address from, uint256 tokenId, bytes calldata data) external returns (bytes4);
-    external returns (bytes4);
 }