another minor edit for BP11

Ho Clemens, I've just been talking to Andrea about BP8. We found an email
where you suggest that BP11 is amended to refer to the discussion of
geometry simplification in BP8 (that Andrea will draft soon).

I just thought I'd flag that I've added this to the relevant section of the
BP sprint plan [1] so that we (I!) don't forget.

Jeremy

[1]:
https://www.w3.org/2015/spatial/wiki/Detailed_planning_BP_document#BP11:_Expose_spatial_data_through_.27convenience_APIs.27