Update index.js

Author: alesanchezr

exercises/03-sum-values/index.js

@@ -1,4 +1,5 @@
-window.onLoad = function calculateSumListener(){
+// adding the function to the window makes it globally available
+window.calculateSumListener = function() {
 	//Return the value of the input #firstNumber
 	var stringA = document.getElementById("firstNumber").value;
 	//Return the value of the input #secondNumber