netfilter: nf_ct_pptp: Fix comment referring to incorrect RFC

The header of this file cites to "RFFC2673" which is "Binary Labels in the
Domain Name System". It should refer to "RFC 2637" which is "Point-to-Point
Tunneling Protocol (PPTP)". This patch also corrects the typo RFFC.

Signed-off-by: Reese Moore <ram@vt.edu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>