doc: fix home dir path in common.conf

* doc/examples/common.conf: fix home dir path

Fix a few typos in user-specific path of common.conf added in d13c5bc24
(gpg,gpgsm: Move use-keyboxd to the new conf file common.conf,
2021-04-19). The file is in the GnuPG home dir. Replace 'use if' with
'use of' as well.

• Signed-off-by: Todd Zullinger <tmz@pobox.com>