index.html: Add image credit

Author: mkoeppe

index.html

@@ -20,5 +20,6 @@ <h1 style="font-size: 5.9vw";>
     </div>
     <div style="position: absolute; top: 8px; left: 16px; ">passagemath</div>
   </h1>
+  Image credit: Z12_128 courtesy <a href="https://www.instagram.com/astrid_koeppe/">Astrid K&ouml;ppe</a>
 </body>
 </html>