fix a minor issue.

index.html

@@ -536,7 +536,7 @@ <h4 class="modal-title custom-modal-title">Preferences</h4>
           </div>
 
           <div class="form-group">
-            <label class="control-label col-sm-6 col-md-6 generic-flex" for="optimalLineLength">
+            <label class="control-label col-sm-6 col-md-6 generic-flex">
               Optimal line length
               <a href="#" data-toggle="tooltip" title="Optimal line length refers to the ideal number of characters (including spaces) per line of text that maximizes readability and comfort for readers.">
                 <img src="img/tooltip.svg" alt="Tooltip Icon" />