cmd/godoc: remove extra / in paths

cmd/godoc: remove extra / in paths

If I click on links which should send you to source code (e.g. type, function, etc),
the link is to //src/... (instead of /src/...).
This causes a DNS resolution failure on the browser.

Quick fix is to remove the leading / from package.html
(since godoc.go src links automatically add a leading / as necessary).

Fixes #3193.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5730059