Update manual page

Author: rocky

images/Mathic3-manual-page.png

@@ -32,8 +32,8 @@ <h1><img id="logo" src="images/logo-heptatom.svg" alt="Mathics logo" height="50p
           </div>
 	  <div class="image">
 	    <a href="#doc">
-              <img src="images/mathics-manual.png" height="300px" width="300px"
-		   alt="Mathics manual screenshot" /></a>
+              <img src="images/Mathics3-manual-page.png" height="300px" width="300px"
+		   alt="Mathics3 manual page screenshot" /></a>
 	  </div>
 	</div>
       </header>