godoc: tiny bug fix - use correct filename when comparing files against the index whitelist
godoc: tiny bug fix - use correct filename when comparing files against the index whitelist

This bug prevented files such as READMEs etc. from being included in the index.
For instance, now author names recorded in the AUTHORS file can be found with
a godoc query.

R=rsc, r2
CC=golang-dev
http://codereview.appspot.com/4005047

wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old