Update link to final ERC-20

contracts/eip20/EIP20Interface.sol

@@ -1,5 +1,5 @@
 // Abstract contract for the full ERC 20 Token standard
-// https://github.com/ethereum/EIPs/issues/20
+// https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md
 pragma solidity ^0.4.18;