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

Commit f5b40aa0dea6 modifies the -C command line option
for rpc.tlsclntd to use the TLS 1.3 SSL_CTX_set_ciphersuites().

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

This is a content change.

Reviewed by: jhb, pauamma_gundo.com (same change for rpc.tlsservd.8)
MFC after: 2 weeks