docs: Correct database_flags syntax for Postgres servers (PSKD-1437)

[dhoucgitter]

Changes

• Update the example postgres servers database_flags value to fix the invalid syntax and include two working example name/value pairs that can be used to set database_flags correctly.

Tests

• After duplicating the error message noted in the internal ticket with the existing documentation format for database_flags, I updated the database_flags value for the default postgres server instance to use the format below that includes commas between name/value pairs and between each database_flags entry as required.

postgres_servers = {    
  default = {    
    database_flags         = [{ name = "cloudsql.enable_pg_cron", value = "true"}, { name = "cloudsql.enable_pgaudit", value = "true"}] 
  },    
}     

• I checked the created postgres server instance in the GCP console to verify that both the cloudsql.enable_pg_cron and cloudsql.enable_pgaudit database flags were set to "on" as expected.