Update "Learn more" href in timetracking template.

app/assets/javascripts/issuable_time_tracker.js.es6

@@ -177,7 +177,7 @@
                   <code>/spend</code>
                   will update the sum of the time spent.
                 </p>
-                <a class='btn btn-default learn-more-button' href='http://example.com/time-tracking-url'> Learn more </a>
+                <a class='btn btn-default learn-more-button' href='https://docs.gitlab.com/ee/workflow/time_tracking.html'> Learn more </a>
               </div>
             </div>
           </div>