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

Commit 0637b12b13be 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.

(cherry picked from commit b4fa7fde15854a7d40ca66694d6589fd45060050)