fix: use correct syntax for RETURNING in query

[mburri]

Using the proposed code snippet in 25_reactive_programming.md#359 resulted in an exception being thrown:

Details:
        Error id 291c8a46-0a80-41b7-99d8-6d9287691027-8, java.util.NoSuchElementException: Column id does not exist
Decorate (Source code):
        Exception in SensorMeasurement.java:26
          24  
          25      public SensorMeasurement(Row row) {
        → 26          this.id = row.getLong("id");
          27          this.data = row.getDouble("data");
          28          this.time = Instant.from(row.getOffsetDateTime("time"));
Stack:
        java.util.NoSuchElementException: Column id does not exist
        at io.vertx.sqlclient.Row.getLong(Row.java:125)
        at io.vertx.mutiny.sqlclient.Row.getLong(Row.java:138)
        at org.acme.entity.SensorMeasurement.<init>(SensorMeasurement.java:26)
        at org.acme.entity.SensorMeasurement.lambda$save$2(SensorMeasurement.java:58)

The actual solution in the /solutions folder does not throw the exception, but it does not return the new id of the SensorMeasurement.

The list of columns in RETURNING should not be placed in paranthesis.
See https://www.postgresql.org/docs/current/dml-returning.html
(unfortunatly - it has no example with multiple columns)