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there wasn't is out of date.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 <[email protected]>

Author: matloob

design/17280-profile-labels.md

@@ -82,9 +82,6 @@ the standard library.
 
     // ProfileLabels is an immutable map of profiler labels. A nil
     // *ProfileLabels is an empty map of labels.
-    // There is intentionally no way to access the profile labels contained
-    // inside the ProfLabels because doing so could create a goroutine-local
-    // storage mechanism.
     type ProfileLabels struct { /* runtime-internal unexported fields */ }
 
     // SetProfileLabels associates the specified profile