one-word manpage correction: snapshot->rollback (#16294)

This commit fixes what is probably a copy-paste mistake. The
`dracut.zfs` manpage claims that the `bootfs.rollback` option executes
`zfs snapshot -Rf`. `zfs snapshot` does not have a `-R` option. `zfs
rollback` does.

Signed-off-by: Alphan Yılmaz <alphanyilmaz@gmail.com>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>

man/man7/dracut.zfs.7

@@ -154,7 +154,7 @@ defaults to the current kernel release.
 .
 .It Sy bootfs.rollback Ns Op Sy = Ns Ar snapshot-name
 Execute
-.Nm zfs Cm snapshot Fl Rf Ar boot-dataset Ns Sy @ Ns Ar snapshot-name
+.Nm zfs Cm rollback Fl Rf Ar boot-dataset Ns Sy @ Ns Ar snapshot-name
 before pivoting to the real root.
 .Ar snapshot-name
 defaults to the current kernel release.