pdns: Fix pdnsutil cross-references

pdns: Fix pdnsutil cross-references

Sphinx reports unknown document warnings when building html-docs:

docs/backends/generic-sql.rst:104: WARNING: unknown document: pdnsutil
docs/backends/geoip.rst:94: WARNING: unknown document: pdnsutil
docs/changelog/4.0.rst:398: WARNING: unknown document: pdnsutil
docs/changelog/4.0.rst:420: WARNING: unknown document: pdnsutil
docs/changelog/4.1.rst:3: WARNING: unknown document: pdnsutil
docs/changelog/4.1.rst:3: WARNING: unknown document: pdnsutil
docs/changelog/4.1.rst:1: WARNING: unknown document: pdnsutil
docs/changelog/4.7.rst:15: WARNING: unknown document: pdnsutil
docs/changelog/4.7.rst:15: WARNING: unknown document: pdnsutil
docs/changelog/4.7.rst:16: WARNING: unknown document: pdnsutil
docs/changelog/4.7.rst:9: WARNING: unknown document: pdnsutil
docs/guides/basic-database.rst:80: WARNING: unknown document: pdnsutil
docs/guides/basic-database.rst:80: WARNING: unknown document: pdnsutil
docs/guides/basic-database.rst:80: WARNING: unknown document: pdnsutil
docs/running.rst:114: WARNING: unknown document: pdnsutil
docs/upgrading.rst:58: WARNING: unknown document: pdnsutil

Fix references to pdnsutil(1) manpage.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>