doc: Fix typos.

* doc/guix.texi (Build Systems)[ocaml-build-system]: Fix typo.
(Log Rotation)[periodic-rotations]: ditto.
(Database Services)[redis-service-type]: ditto.
(OpenSMTPD Service)[opensmtpd-configuration]: ditto.
(VPN Services)[OpenVPN]: ditto.
(Power management Services)[tlp-configuration]: ditto.
(Git daemon service)[git-daemon-service]: ditto.
(Running GuixSD in a VM): ditto.
* gnu/services/pm.scm (tlp-configuration)[runtime-pm-blacklist]: Fix typo.
* gnu/services/vpn.scm (openvpn-client-configuration)[status]: ditto.