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

Commit 72bf76d6b8c9 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.

(cherry picked from commit b387a075d9db81a7bcc3adcc7c1c5ba93e8d8f66)