pallet_aura docs still refer to frame_consensus

[JoshOrndorff]

The module documentation for pallet_aura (rendered here) links to frame_consensus which I believe has been removed for quite a while.

Perhaps the line should just be removed, or maybe the link should be pointed at pallet_babe instead.

[bkchr]

There are other occurrences of frame_consensus in the other pallets. These need to be removed as well.