go2draft-contracts.md: update to new contracts draft design
go2draft-contracts.md: update to new contracts draft design

The design of contracts is simplified significantly.

Change-Id: I16aba65fa09d7e7fee3e2aeeabe1e76c7c8c75af
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/187740
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

golang/proposal,golang/proposal