go2draft-contracts.md: add missing period
go2draft-contracts.md: add missing period

Change-Id: Ibde4ea7ab98d795c1504961d02d25ef4b4b1c12e
GitHub-Last-Rev: 020bcd096aa288cc2314f4e14d87b5fe0cef523d
GitHub-Pull-Request: golang/proposal#20
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/187961
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/proposal,golang/proposal