Fix iframe source path in index.html

Author: HendrikBorgelt

synthesis/index.html

@@ -2166,7 +2166,7 @@ <h2 id="slots">Slots</h2>
 <p><strong>Schema Reference:</strong> <a href="../elements/sample_pretreatment/">sample_pretreatment</a></p>
 </details>
 <p><iframe
-    src="assets/chart_synthesis.html"
+    src="./assets/chart_synthesis.html"
     width="100%"
     height= "800vh"
     style="border: 2px solid #5C88DA; background-color: #F0F8FF;
@@ -2232,4 +2232,4 @@ <h2 id="slots">Slots</h2>
 
 
   </body>
-</html>
+</html>