runtime/pprof: Fix type name in function comment

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>

src/runtime/pprof/label.go

@@ -52,7 +52,7 @@ func WithLabels(ctx context.Context, labels LabelSet) context.Context {
 }
 
 // Labels takes an even number of strings representing key-value pairs
-// and makes a LabelList containing them.
+// and makes a LabelSet containing them.
 // A label overwrites a prior label with the same key.
 func Labels(args ...string) LabelSet {
 	if len(args)%2 != 0 {