server/prpc: fix helloworld link
server/prpc: fix helloworld link

Github link != package path

TBR=iannucci@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1592933002

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go