security/obfs4proxy-tor: Update pkg-message

Update pkg-message to reflect obfs4's man page, obfs4 works only as managed
pluggable transport spawned as a helper process via the tor daemon, and needs
no extra options to operate in such mode.

Also fix a typo and a white space.

PR: 225068
Submitted by: Vinícius Zavam <egypcio@googlemail.com> (maintainer)