Fix output of aws_db_instance port via endpoint

When trying to output the `endpoint` attribute of an `aws_db_instance`,
the port was not being emitted properly. Converting the port number from
an integer to a string has resolved the issue.

```
Apply complete! Resources: 2 added, 0 changed, 0 destroyed.

The state of your infrastructure has been saved to the path
below. This state is required to modify and destroy your
infrastructure, so keep it safe. To inspect the complete state
use the `terraform show` command.

State path: terraform.tfstate

Outputs:

  azavea_postgresql_01_endpoint = azavea-postgresql-01.c5fivu7nazve.us-east-1.rds.amazonaws.com:5432
```