go.tools/go/types: fix typo in comment
go.tools/go/types: fix typo in comment

TBR=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/135960043

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