rpc.tlsclntd: Add the -2 option to the usage line

Commit 72bf76d6b8 added the -2 option to use
TLS 1.2 NFS-over-TLS mounts.  It did not add -2
to the usage message.

This patch adds -2 to the usage message.

MFC after:	2 weeks

usr.sbin/rpc.tlsclntd/rpc.tlsclntd.c

@@ -188,6 +188,7 @@ main(int argc, char **argv)
 			break;
 		default:
 			fprintf(stderr, "usage: %s "
+			    "[-2/--usetls1_2] "
 			    "[-C/--ciphers available_ciphers] "
 			    "[-D/--certdir certdir] [-d/--debuglevel] "
 			    "[-l/--verifylocs CAfile] [-m/--mutualverf] "