Update header for the rename of the Expert Trainer's Class
<div class="dropdown-menu">
<a class="dropdown-item" href="http://appinventor.mit.edu/about-us">About App Inventor</a>
<a class="dropdown-item" href="http://appinventor.mit.edu/explore/our-team">Our Team</a>
- <a class="dropdown-item" href="http://appinventor.mit.edu/explore/master-trainers">Master Trainers</a>
+ <a class="dropdown-item" href="http://appinventor.mit.edu/explore/expert-trainers">Expert Trainers</a>
<a class="dropdown-item" href="http://appinventor.mit.edu/explore/app-month-gallery">App of the Month</a>
<a class="dropdown-item"
href="http://appinventor.mit.edu/ai2/ReleaseNotes">Release Notes</a>
