minor patch to fetchall_hashref POD

[Jeff Zucker]

(diff for DBI.pm 1.30)

4516c4516
<   $hash_ref = $dbh->fetchall_hashref($key_field);
---
 >   $hash_ref = $sth->fetchall_hashref($key_field);
4544a4545
 > This method can not be used if any of the key field values is NULL.

-- 
Jeff