Correct docstring for plot.py (#539)

When running the latest version of the code, the example didn't run
without adding these two arguments.

Author: tompollard

wfdb/plot/plot.py

@@ -238,6 +238,7 @@ def plot_items(
 
     >>> wfdb.plot_items(signal=record.p_signal,
                         ann_samp=[ann.sample, ann.sample],
+                        fs=record.fs, sig_units=record.units,
                         title='MIT-BIH Record 100', time_units='seconds',
                         figsize=(10,4), ecg_grids='all')