Fix doc in Postgres database creation

By default encoding for new Postgres database is [utf8](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb#L57) and it has [assert](https://github.com/aderyabin/rails/blob/54a2ec13026b14da70f1894d87a4fe04a63e5828/activerecord/test/cases/adapters/postgresql/active_schema_test.rb#L17) but not pointed in doc.