rgw: Fix typo in rgw_user.cc

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
Merge pull request #437 from kri5/wip-fix-typo-rgw

rgw: Fix typo in rgw_user.cc

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>