The name LabelList was changed to LabelSet during the development of the
proposal [1], except in one function comment. This commit fixes that.

Fixes #20905.

[1] https://github.com/golang/go/issues/17280

Change-Id: Id4f48d59d7d513fa24b2e42795c2baa5ceb78f36
Reviewed-on: https://go-review.googlesource.com/47470Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>