Fix guide index's link to custom form

docs/guides/src/asciidoc/index.asc

@@ -12,4 +12,4 @@ These are the most basic starting points for using a web based application.
 
 == Simple Customization
 
-* link:hellomvc.html[Creating a custom login form] - demonstrates how to create a custom login form
+* link:form.html[Creating a custom login form] - demonstrates how to create a custom login form