compress/lzw: fix comment re high-code invariant

compress/lzw: fix comment re high-code invariant

The listed invariant, while technically true, was misleading, and the
invariant can be tightened. We never actually get to (d.hi ==
d.overflow), due to the "d.hi--" line in the decoder.decode method.

This is a comment-only commit, changing the comment to match the code.

A follow-up commit could restore the comment, changing the code to match
the original intented invariant. But the first step is to have the
comment and the code say the same thing.

Change-Id: Ifc9f78d5060454fc107af9be298026bf3043d400
Reviewed-on: https://go-review.googlesource.com/c/go/+/191358
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>