Fix broken link

Author: rocky

index.html

@@ -217,8 +217,8 @@ <h2 id="doc">Documentation</h2>
 	  <p>
 	    <div class="image">
 	      <a href="https://mathics.org/docs/mathics-latest.pdf">
-		<img src="images/mathics-manual.png"
-                     alt="Mathics manual screenshot" /></a>
+		<img src="images/Mathics3-manual-page.png"
+                     alt="Mathics3 manual screenshot" /></a>
             </div>
 	  </p>
         </div>