Improve DB names in examples

[jrodewig]

Problem

This is strictly a QoL improvement. No real problem here.

• When using fauna database create/delete, the relationship can get confusing between child and parent DBs, particularly when using a --secret.
• Using example as the default database name is fine but it's not descriptive.

Solution

• Where applicable, use child_db and parent_db in examples so it's more obvious.
• Use the shorter but more descriptive my_db as the default database name. The name also indicates that underscores are supported in database names.