Add <script> tag

Author: lulunac27a

index.html

@@ -6,4 +6,5 @@ <h1>Sum of Digits Calculator</h1>
     <input type="number" id="numberInput">
     <button onclick="calculateSumOfDigits()">Calculate</button>
     Sum of Digits: <p id="result"></p>
+    <script src="app.js"></script>
 </body>