fix link to background example

Author: mathisonian

site-header.html

@@ -11,7 +11,7 @@ <h1>Euclid JS</h1>
       <a href="parse.html">Parsing from a geometry-proof like description.</a>
     </li>
     <li>
-      <a href="index.html">Just for fun: as a dynamic background</a>
+      <a href="background.html">Just for fun: as a dynamic background</a>
     </li>
     <li>
       <a href="https://github.com/anandthakker/euclid">Source on Github</a>