microtiming.normalize_onsets correct docstring

[mrocamora]

Remove "(in seconds)" from the following description. The onsets return are beat normalized.

Returns
-------
onsets_normalized : list of np.ndarray
normalized onsets (in seconds) as a list of arrays, one element for each instrument

Check the description of the list of np.ndarray returned. It is actually a list of lists, each of the list corresponds to a different instrument, and the lists contain arrays with the beat normalized onsets.