Maximum frequency choices in the examples

@avi.vajpeyi in the 150914 example, here the PSD only goes up to 1024Hz, but the data is sampled at 4096Hz with a maximum frequency of 2048Hz. I think in this case bilby just ignores the context without a PSD. But, for the sake of the example, it may be worth adding a line maximum_frequency=1024 which will ensure that the maximum frequency is set to 1024