doc: remove invalid hash in link

Sections in the https://tools.ietf.org/html/rfc1122 are not linkable
anymore. Replace the possibly confusing hash with the explicit
section number in the text.

PR-URL: https://github.com/nodejs/node/pull/15542


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>