Fedora version updated in Devstack doc
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 <60e5f096d6381fcaf6cafad95090e459284265d6@gmail.com>

group-policy/devstack,dtroyer/devstack,group-policy/devstack,noironetworks/devstack,openstack-dev/devstack,nawawi/openstack,dtroyer/devstack,noironetworks/devstack,nawawi/openstack,openstack-dev/devstack