ctr username parameter typo

[jan-ludvik]

I was trying to download couple of images from private registries and it was failing on authentication.
It was resolved when I changed username parameter for ctr from --u to -u - at this line: https://github.com/aws-samples/bottlerocket-images-cache/blob/main/snapshot.sh#L217

Based on ctr manpage it's ether -u or --user:

--user, -u="": user[:password] Registry user and password