Escape CDATA element (#10377)

documentation/advanced/advanced-embedding.asciidoc

@@ -136,7 +136,7 @@ you should check that the bootstrap script was loaded properly.
 
 [subs="normal"]
 ----
-<script type="text/javascript">//<![CDATA[
+<script type="text/javascript">//<!\[CDATA[
   if (!window.vaadin)
       alert("[replaceable]#Failed to load the bootstrap JavaScript:#"+
             "[replaceable]#VAADIN/vaadinBootstrap.js#");