extension "subnet_dns_publish_fixed_ips" named wrong in the docs

[Florian 'rephlex' Panzer]

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [x] This doc is inaccurate in this way: Extensions true name is "subnet_dns_publish_fixed_ip", the docs say "subnet_dns_publish_fixed_ips".

- [ ] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

-----------------------------------
Release: 16.0.0.0b2.dev135 on 2019-12-03 15:27:53
SHA: 1b33ed164b23757f5578dfb51f6ebf0e8e01af7a
Source: https://opendev.org/openstack/neutron/src/doc/source/admin/config-dns-int-ext-serv.rst
URL: https://docs.openstack.org/neutron/latest/admin/config-dns-int-ext-serv.html

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/716072

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/716072
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=6d619ea7c13e89ec575295f04c63ae316759c50a
Submitter: Zuul
Branch: master

commit 6d619ea7c13e89ec575295f04c63ae316759c50a
Author: Slawek Kaplonski <email address hidden>
Date: Mon Mar 30 22:10:05 2020 +0200

    Fix name of "subnet_dns_publish_fixed_ip" extension in docs

    Correct name of the extension is "subnet_dns_publish_fixed_ip" but
    in the Neutron docs it was "subnet_dns_publish_fixed_ips".

    Change-Id: I52e313766d08879b8163b36f41515ce4afd5c470
    Closes-Bug: #1869057