Fedora version is outdated

[Victoria Martinez de la Cruz]

Docs say that you require Fedora 24/25 to run Devstack, but Devstack is working in newer versions. Update document to say that Fedora 28 can be used instead.

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:

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

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-----------------------------------
Release: 0.0.1.dev9325 on 2018-10-10 15:36
SHA: b138df3d39cb838e274416d430f4949cbfb723a9
Source: https://git.openstack.org/cgit/openstack-dev/devstack/tree/doc/source/index.rst
URL: https://docs.openstack.org/devstack/latest/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/609835

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/609835
Committed: https://git.openstack.org/cgit/openstack-dev/devstack/commit/?id=1c75d2f06c03b0ed0517b499674d36052bcc3793
Submitter: Zuul
Branch: master

commit 1c75d2f06c03b0ed0517b499674d36052bcc3793
Author: Laura Sofia Enriquez <email address hidden>
Date: Thu Oct 11 19:13:17 2018 -0300

    Fedora version updated in Devstack doc

    Docs say that you require Fedora 24/25 to run Devstack, but Devstack
    is working in newer versions. Update document to say that Fedora 28
    can be used instead.

    Closes-Bug: #1797239

    Change-Id: Ie5227db9943e5ddb93cd37440165eabbae22f4fc
    Signed-off-by: Laura Sofia Enriquez <email address hidden>