sctp: fix man page for socket option controlling delayed acks

The SCTP_DELAYED_ACK_TIME socket option was replaced by the
SCTP_DELAYED_SACK in the socket API specification in
draft-ietf-tsvwg-sctpsocket-14.
The code was updated in r170056, but the man page was not.

PR: 272124
MFC after: 3 days