doc: Change id of footer

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

doc/style.css

@@ -150,7 +150,7 @@ div#plusone {
 	float: right;
 }
 
-div#copyright {
+div#footer {
 	color: #666;
 	font-size: 14px;
 	margin: 40px 0;

lib/godoc/godoc.html

@@ -50,7 +50,7 @@
 
 </div>
 
-<div id="copyright">
+<div id="footer">
 Build version {{html .Version}}.
 Except as noted, this content is licensed under a
 Creative Commons Attribution 3.0 License.