go.tools/go/importer: fix format string
go.tools/go/importer: fix format string

R=adonovan
CC=golang-codereviews
https://golang.org/cl/52360045

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