Update jspsych-extension-mouse-tracking.html

The const `start` has been removed in the previous commit, and does not seem to be currently required to run the demo (i.e., the reference to it raises "Uncaught ReferenceError:".

examples/jspsych-extension-mouse-tracking.html

@@ -84,7 +84,7 @@
   };
 
   if (typeof jsPsych !== "undefined") {
-    jsPsych.run([start, trial_loop]);
+    jsPsych.run([trial_loop]);
   } else {
     document.body.innerHTML = '<div style="text-align:center; margin-top:50%; transform:translate(0,-50%);">You must be online to view the plugin demo.</div>';
   }