Update docs to reflect default database name

[pdgonzalez872]

Description of changes:

Folks may not remember how overwhelming it is to get started with AWS. In hindsight, after a couple of Google searches for the error you do eventually find a solution.

When creating the RDS instance you are prompted for a database name. This name turns out to be the instance name, not the database name. When using the commands for psql you will get an error along the lines of "the database name does not exist", which is in fact, true.

A database is created by default when there is no additional configuration and its name is postgres. I had this issue and using postgres as dbname solved my problem.

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[hamxabaig]

Just wasted an hour on this, this will be a good addition to the docs.

[John-Shirley]

Thank you for telling us about this issue and I am sorry about your frustration with the documentation.
A member of the team will be working on it and will provide updates here about progress.

[pdgonzalez872]

@John-Shirley no worries! Thanks for responding. In my case, the change I suggest here solved my problem and I believe it is enough to help a lot of other folks that didn't surface the issue.

Hope it gets merged soon. Thanks again, Paulo.