Made tutorial 15 compatible with the latest version of Solidity

Author: James Lockhart

tutorial-15/ExternalContract.sol

@@ -1,11 +1,11 @@
-pragma solidity ^0.4.0;
+pragma solidity ^0.5.0;
 
 contract ExternalContract {
-    function externalCall(string x) external returns (uint) {
+    function externalCall(string calldata x) external returns (uint) {
         return 123;
     }
 
-    function publicCall(string x) public returns (uint) {
+    function publicCall(string memory x) public returns (uint) {
         return 123;
     }
-}
+}