cmd/fetchlogs: fix cache location on OS X

OS X uses $HOME/Library/Caches instead of $HOME/.cache.

Also make the copyright notice not a doc comment,
which is the real reason I opened this file.

Change-Id: I3af75465a3831aed4d38e2c667c813440e20d0e1
Reviewed-on: https://go-review.googlesource.com/19000
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>