Update index.html

Added "previous" button.

index.html

@@ -24,6 +24,7 @@ <h2 id="question-title"></h2>
         <div id="question-text"></div>
         <form id="answers-form"></form>
         <p id="question-number"></p>
+        <button id="prevQuestionBtn">Previous</button>
         <button id="skipQuestionBtn">Skip</button>
         <button id="nextQuestionBtn">Next</button>
     </div>