doc/go1.7.html: html tidy

Change-Id: I0e07610bae641cd63769b520089f5d854d796648
Reviewed-on: https://go-review.googlesource.com/23770
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>