rpc.tlsservd: Update the man page for the -C option

Commit 712aac1389e8 adds a new -C command line option
to rpc.tlsservd.

This patch updates the man page for this new command
line option.

This is a content change.

Reviewed by: jhb, pauamma_gundo.com
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35136