filtering tutorial needs attention

[drammock]

Proposed documentation enhancement

In the "background on filtering" tutorial, the "Power line noise" section used to show all 3 sensor types, now it only shows mag because at some point we added .pick(['mag', 'stim']) at the beginning of the tutorial. This makes both the prose and the code not really make sense, e.g., the code loops over fig.axes but there is only one axes shown.