getting-started: Fix path to firecracker binary
getting-started: Fix path to firecracker binary

The downloadable tarball is now in a release-${latest}-$(uname -m)
subdirectory.

Signed-off-by: Colin Percival <a9cd60ed1b2f93c64c71ff58c35e59347a22e2f3@FreeBSD.org>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker