content: fix URLs in web service exercise

Update golang/go#13012

Change-Id: I62277ba3c8bcb57c8e8da5d00faec77c2a1ffa04
Reviewed-on: https://go-review.googlesource.com/17162
Reviewed-by: Katrina Owen <katrina.owen@gmail.com>
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed