dbm_delete(3) correct man page to match current behaviour.

"The dbm_store() and dbm_delete() functions shall return 0 when they
succeed and a negative value when they fail."

Reference:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/dbm_clearerr.html

PR: 42422
Suggested by: delphij
MFC after: 3 days