encoding/xml: fix doc comment
encoding/xml: fix doc comment

The tag is ",chardata" not "chardata".

Fixes issue 6631.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/19300046

pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme