Fix a grammatical error in a comment (#2352)

changed a comment in the isContract function to make it grammatically right.

contracts/utils/Address.sol

@@ -24,7 +24,7 @@ library Address {
      * ====
      */
     function isContract(address account) internal view returns (bool) {
-        // This method relies in extcodesize, which returns 0 for contracts in
+        // This method relies on extcodesize, which returns 0 for contracts in
         // construction, since the code is only stored at the end of the
         // constructor execution.