Add missing space to checksum string

[gldraphael]

1. Added the missing space to the checksum string for the check to pass.
2. Add w flag to sha256sum so that user is warned if a formatting issue (like the missing space) occurs again in the future.

[natemcmaster]

Thanks for the PR. I don't see a description of the problem you're solving. I ran the original instructions and don't see a problem, even when I added --warn to sha256sum. Can you explain what problem you encountered? I'd like to validate that these changes work before merging.

[natemcmaster]

[gldraphael]

Hey Nate, neither can I reproduce this anymore nor find the failing builds from build history (they've been deleted). I'm guessing the base images have been updated upstream to use a newer sha256sum that's more forgiving of the spaces -- which if true is a good thing IMO.

(I picked the checksum with the additional space from the nodejs website.)

[natemcmaster]

Ok. This is just sample code. I suppose someone out there might have an older version of sha256sum, so I'll merge this. Thanks for the contribution!