Polina Goltsman issue 12

"sec. 3.1
========
old:
   o  To determine its position in the overlay topology (if the overlay
      is structured; topology plugins do not need to be structured).
new:
   o  To determine its position in the overlay topology (if the overlay
      is structured; overlays do not need to be structured).

a structured topology plugin is not the same as a structured overlay"

- As suggested.