doc: Fix typos.

* doc/guix.texi (rakudo-build-system, libvirt-configuration,
pam-krb5): Missing period between sentences.
(gnu services vpn): Fix server spelling.
(agetty-configuration): Missing subject in the sentence.
(udev-service): Fix markup for Scheme procedures.
({gnome,xfce,mate}-desktop-service): Missing colon after default.
---
 doc/guix.texi | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)