doc: rgw: el6 documentation fixes

- fix a couple of typo for repo configuration and service restart
- the ceph package must be installed on RPM distro since the init
script relies on ceph-conf
- Note on radosgw service name for RPM distro

Signed-off-by: Alexandre Marangone <alexandre.marangone@inktank.com>
Merge pull request #1174 from alram/master

doc: rgw: el6 documentation fixes