Correct zfs-send(8) on readonly sends

zfs-send(8) claimed in the flags list you could use -pR when sending
a readonly filesystem or volume. You cannot.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12336