exp/html: remove "INCOMPLETE" comment

I think that the parser is complete enough to take that warning out.
It passes the test suite.
There may be incompatible API changes, but being in the exp directory
is warning enough for that.

R=nigeltao
CC=golang-dev
https://golang.org/cl/7131050

src/pkg/exp/html/doc.go

@@ -4,7 +4,6 @@
 
 /*
 Package html implements an HTML5-compliant tokenizer and parser.
-INCOMPLETE.
 
 Tokenization is done by creating a Tokenizer for an io.Reader r. It is the
 caller's responsibility to ensure that r provides UTF-8 encoded HTML.