The heading "Copyright" uses id="copyright" as the anchor name.
However, there is also a <div id="copyright"> at the bottom.
Using the same ID value twice in the same file is illegal
(http://www.w3.org/TR/html401/struct/links.html#h-12.2.3).

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5909045