html: fix typo in package docs.

Fixes #2419.

R=dsymonds, rsc
CC=golang-dev
https://golang.org/cl/5352046

src/pkg/html/doc.go

@@ -61,7 +61,7 @@ call to Next. For example, to extract an HTML page's anchor text:
 		case StartTagToken, EndTagToken:
 			tn, _ := z.TagName()
 			if len(tn) == 1 && tn[0] == 'a' {
-				if tt == StartTag {
+				if tt == StartTagToken {
 					depth++
 				} else {
 					depth--