Fixed ``boto.connect_rds2()``.

The rest of ``rds2`` should be fine, but I copy-pasted the wrong style ``boto.connect_FOO`` & without this, establishing a connection this way fails. Manually importing the ``RDSConnection`` or using ``boto.rds2.connect_to_region`` works as expected.

This was added in Boto 2.26.0. Is it worth a patch release?

Review please? @danielgtaylor