cmd/compile: repair copy-pasted method comment

Inadvertent copy-pasta in CL
https://go-review.googlesource.com/c/36207/

Is this allowed for 1.9 post-freeze?

Change-Id: I00dd27f9f53babeded3695346986a7a491f8a771
Reviewed-on: https://go-review.googlesource.com/43092


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>