design/2981: fix json tags
design/2981: fix json tags

Change-Id: I136dd59217428bbb9a70dcd0bedd5188dccf8729
Reviewed-on: https://go-review.googlesource.com/30490
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/proposal,golang/proposal