Update Fedora zfs-release RPM link to '2-8'

Update Fedora zfs-release RPM link from '2-6' to '2-8'.

Author: tonyhutter

docs/Getting Started/Fedora/index.rst

@@ -24,7 +24,7 @@ see below.
 
 #. Add ZFS repo::
 
-    dnf install -y https://zfsonlinux.org/fedora/zfs-release-2-6$(rpm --eval "%{dist}").noarch.rpm
+    dnf install -y https://zfsonlinux.org/fedora/zfs-release-2-8$(rpm --eval "%{dist}").noarch.rpm
 
    List of repos is available `here <https://github.com/zfsonlinux/zfsonlinux.github.com/tree/master/fedora>`__.