s/thread/thead/ as this is Table HEAD and not a thread as indicated by
the closing tag an context this apears in.

Change-Id: I3aa0cc95b96c9a594cb5a49713efa22d67e4990c
Reviewed-on: https://go-review.googlesource.com/112675Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>