zfs-receive.8: fix Op Fl x Ar encryption in running text

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Amanakis <gamanakis@gmail.com>
Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13101