bug#47812: Incorrect code in the Guix manual, but only on the website

[Mark H Weaver <mhw@netris.org>]

In the Guix manual, as it appears on our website,

  https://guix.gnu.org/manual/devel/en/html_node/X-Window.html

the manual entry for 'slim-service-type' suggests the following code to
remove the 'gdm' service from %desktop-services.

--8<---------------cut here---------------start------------->8---
   (modify-services %desktop-services
     (delete gdm-service-type))
--8<---------------cut here---------------end--------------->8---

This code is incorrect.  'modify-services' does not support deleting
services.

The puzzling thing is that I can't find where this suggested code exists
in our git repository.  The manual in our git repository is correct.
Instead of the code above, it suggests:

--8<---------------cut here---------------start------------->8---
   (remove (lambda (service)
             (eq? (service-kind service) gdm-service-type))
           %desktop-services)
--8<---------------cut here---------------end--------------->8---

This is the case on our 'master', 'core-updates', and 'version-1.2.0'
branches.  See:

  https://git.sv.gnu.org/cgit/guix.git/tree/doc/guix.texi?id=ebd021f73292bf47b26a799665d6204574bae239#n17891
  https://git.sv.gnu.org/cgit/guix.git/tree/doc/guix.texi?h=core-updates&id=76fc36d0a7215979bb74c05840f5a4de4ab5ea93#n17756
  https://git.sv.gnu.org/cgit/guix.git/tree/doc/guix.texi?h=version-1.2.0&id=8603bcd5822a1a8364a7a74a698059d3d147568b#n17076

The correct text was added about 2 years ago, in the following commit:

  https://git.sv.gnu.org/cgit/guix.git/commit/?id=dbef9015db107dd148133420b89af552ef08f8ee

I see no evidence of it being changed since then.

Any idea where the incorrect code on the website manual is coming from?

      Thanks,
        Mark