go/analysis/singlechecker: use Stderr in flag.Usage
go/analysis/singlechecker: use Stderr in flag.Usage

It was using a mix of stdout and stderr. Most users won't notice, but
it's inconsistent for no apparent reason. In particular, I noticed as
some of my tool execution tests started failing.

Change-Id: I9afe5f5bed0a575d3ba20e8dc1cc593c35565cf9
Reviewed-on: https://go-review.googlesource.com/c/162717
Run-TryBot: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools