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

Commit 0637b12b13 added the -2 option to allow
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.tlsservd/rpc.tlsservd.c

@@ -237,6 +237,7 @@ main(int argc, char **argv)
 			break;
 		default:
 			fprintf(stderr, "usage: %s "
+			    "[-2/--allowtls1_2] "
 			    "[-C/--ciphers available_ciphers] "
 			    "[-D/--certdir certdir] [-d/--debuglevel] "
 			    "[-h/--checkhost] "