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

lib/libc/db/man/dbm.3

@@ -15,7 +15,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd April 16, 2006
+.Dd February 19, 2015
 .Dt DBM 3
 .Os
 .Sh NAME
@@ -174,9 +174,7 @@ deletes the entry for
 The
 .Fn dbm_delete
 function
-normally returns zero but returns 1 if there was no entry with
-.Fa key
-in the database or returns -1 and sets
+normally returns zero or returns -1 and sets
 .Va errno
 if there were any errors.
 .Pp