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 <lsofia.enriquez@gmail.com>
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 2ff4ff0..fcf1e82 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -39,8 +39,9 @@
 -------------
 
 Start with a clean and minimal install of a Linux system. Devstack
-attempts to support Ubuntu 16.04/17.04, Fedora 24/25, CentOS/RHEL 7,
-as well as Debian and OpenSUSE.
+attempts to support the two latest LTS releases of Ubuntu, the
+latest/current Fedora version, CentOS/RHEL 7, as well as Debian and
+OpenSUSE.
 
 If you do not have a preference, Ubuntu 16.04 is the most tested, and
 will probably go the smoothest.