Fix command in docker login docs
Fix command in docker login docs

the alternative command provided for step 1 did not include the
`--region` parameter, causing it to fail

Signed-off-by: Patrick Roy <739e3d835447c4a33f21e8437b8228702e8d1c04@amazon.co.uk>

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