Fix README snippet typo

(cherry picked from commit 0b8296e52596db8ef97f313fe3507cac8fbbcde3)

README.md

@@ -23,7 +23,7 @@ import 'openzeppelin-solidity/contracts/token/ERC721/ERC721Full.sol';
 import 'openzeppelin-solidity/contracts/token/ERC721/ERC721Mintable.sol';
 
 contract MyNFT is ERC721Full, ERC721Mintable {
-  constructors() ERC721Full("MyNFT", "MNFT") public {
+  constructor() ERC721Full("MyNFT", "MNFT") public {
   }
 }
 ```