Incorrect version used for alpha packages

[wilhen01]

bump-cdk is currently setting incorrect versions for alpha packages.

e.g. @aws-cdk/aws-synthetics-alpha is set to 2.9.0 not 2.9.0-alpha.0.

I think these packages are using -alpha.0 as a standard suffix so this should be a fairly easy fix?

[wilhen01]

I will try and raise a PR for this when I get a second...