fix: frontend main.html, use right closing tag for buttonNext

templates/main.html

@@ -114,7 +114,8 @@
 
             <div id="nextButtonDiv" style="position: absolute; left: 90%; top:0%; height: 100%;" class="flex-container">
                 <button id="buttonNext" class="expanded button-next" onclick="javascript:getNextSample()">
-                    <span></span></a>
+                    <span></span>
+                </button>
             </div>
         </div>
 
@@ -178,4 +179,4 @@
 
 
 
-</html>
+</html>