go.tools/astutil: fix parameter omission in doc comment
go.tools/astutil: fix parameter omission in doc comment

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/98290045

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,chenjianlong/go.tools,karmakaze/go.tools