Fix the link from liberty to mitaka
Fix the link from liberty to mitaka

Link for liberty is changed to
upcoming mitaka design summit

Change-Id: Ia78c35ba244012dec82e917849d88f901cb04318
Closes-Bug: #1490474

Nabs007/training-guides,openstack/training-guides,openstack/training-guides,Nabs007/training-guides,Nabs007/training-guides