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://golang.org/cl/98290045

maddyonline/tools,springning/tools,matryer/tools,maddyonline/tools,golang/tools,argandas/tools,cmars/tools,glasser/golang-tools,dmitris/tools,maddyonline/tools,cw2018/tools,jdhenke/tools,golang/tools,jdhenke/tools,cw2018/tools,cmars/tools,runcom/tools,glasser/golang-tools,runcom/tools,dmitris/tools,josharian/tools,runcom/tools,josharian/tools,golang/tools,cmars/tools,maddyonline/tools,springning/tools,magiccao/tools,golang/tools,4ad/tools,dmitris/tools,magiccao/tools,cmars/tools,syreclabs/go-tools,matryer/tools,springning/tools,magiccao/tools,argandas/tools,syreclabs/go-tools,sshetty10/tools,springning/tools,matryer/tools,golang/tools,argandas/tools,nicot/tools,4ad/tools,sshetty10/tools,dmitris/tools,argandas/tools,jdhenke/tools,sshetty10/tools,4ad/tools,syreclabs/go-tools,sshetty10/tools,glasser/golang-tools,syreclabs/go-tools,glasser/golang-tools,matryer/tools,nicot/tools,runcom/tools,josharian/tools,magiccao/tools,nicot/tools,jdhenke/tools,cw2018/tools,syreclabs/go-tools,cw2018/tools,josharian/tools,nicot/tools,syreclabs/go-tools