improve read_fake_data function in chapter 5 that produces anomalous …

[mdautrey]

…and non-anomalous data

In the example provided with the book, the function read_fake_data produces an anomalous data point only once day which is not consistent with the definition provided in the book (anomalous day = a full day of anomalous data). This PR proposed a modified read_fake_data function that is consistent with the definition.