content: make urls explicit in web server exercise

Some readers became confused because the server doesn't
register a handler for "/", causing them to get a 404
on localhost:4000 even when having completed the exercise
correctly.

Fixes golang/go#13012

Change-Id: I97ba511ca0b38097e5b54b75a7478c7c59912e43
Reviewed-on: https://go-review.googlesource.com/16923
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed