design: minor fixes to 27935-unbounded-queue-package.md design doc.
design: minor fixes to 27935-unbounded-queue-package.md design doc.

Change-Id: Ic046d1cfd42506edf0b4c1d924e6a5b7e6128ee2
Reviewed-on: https://go-review.googlesource.com/c/139217
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/proposal,golang/proposal