fix ssh_retrieve_data testcase for IOC

`FAILED tests/test_observations.py::test_ssh_retrieve_data[ioc] - assert 0 == 1` has been failing for two weeks now. Further investigation shows that this is because station AMTSI was added on 15 august 2025. This station only has data from that date onwards, so not for the default period used in the test. Fix this by not taking the first but the third station