Domains such as a.example.com cannot be used

[bana118]

I tried to create an AWS Hubs Cloud with a domain like "a.example.com" but it failed with an error message saying "Must be a valid domain name (eg myhub.com or hub.mycompany.com)".

Oddly enough, it succeeded for the domain "a..example.com".

I think this is due to a mistake in the AllowedPattern below. (The first \. is unnecessary)

https://github.com/mozilla/hubs-ops/blob/df0d01979f8527dd11dbbaa5bfd47a78e6855331/cloudformation/stack.yaml#L150