cmd/godoc: fix indexing
cmd/godoc: fix indexing

Use the flag.

Thanks to Travis Cline.

Fixes issue 6233

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/13284044

tmc/mirror-go.tools,tmc/mirror-go.tools,t-yuki/go.tools.old,tmc/mirror-go.tools,t-yuki/go.tools.old,t-yuki/go.tools.old