Update old reference to gpinit mirror mode option
Update old reference to gpinit mirror mode option

The example text included the old `-S` syntax for specifying the mirror
mode.

This was updated in this commit:
a398b43724ec2b5818bceeb1204a014a3a80ba7a but the `--help` text of
`gpinitsystem` was missed.

Co-authored-by: Ivan Novick <44250f61e72b49f9c3d7f9df41ef69e228ae5438@pivotal.io>
Co-authored-by: Kris Macoskey <02d17b27bd81bba87d36066114b34c9a266751ac@pivotal.io>

ashwinstar/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,adam8157/gpdb,jmcatamney/gpdb,lisakowen/gpdb,lisakowen/gpdb,ashwinstar/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,jmcatamney/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,ashwinstar/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,50wu/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb