Dead link in documentation

[Dave Atkinson]

Dear Lilypond Devs,

I'm reading Lilypond - Extending, and trying to learn a bit of Scheme to 
achieve my aims. At the bottom of the page

https://lilypond.org/doc/v2.19/Documentation/extending/scheme-simple-data-types

there is the following sentence:

There are additional Scheme data types that are not discussed here. For 
a complete listing see the Guile reference guide, 
http://www.gnu.org/software/guile/manual/html_node/Simple-Data-Types.html.

The link appears to be dead and takes me to a 404 - Page Not Found 
landing page at gnu.org.

Cheers,

Dave