Fix code smells in exercise.html

frontend/www/exercise.html

@@ -51,7 +51,7 @@
             </div>
         </div>
             <table id="leaderboardstable" class="table table-striped">
-                <tr><th>Rank<th>Username<th>Score
+                <tr><th scope="col">Rank<th scope="col">Username<th scope="col">Score
             </table>
           </div>
         <script src="scripts/defaults.js"></script>