design: update profile labels proposal comment on ProfileLabels
design: update profile labels proposal comment on ProfileLabels

There now is a way to access labels, so the previous comment saying
there wasn't is out of date.

Change-Id: I855f42276083b1c236fdc88fddd4504d90107347
Reviewed-on: https://go-review.googlesource.com/31615
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/proposal,golang/proposal