include NaN in value counts code example

Example 1: count values pandas

>>> index = pd.Index([3, 1, 2, 3, 4, np.nan])
>>> index.value_counts()
3.0    2
4.0    1
2.0    1
1.0    1
dtype: int64

Example 2: relativefrequencies of the unique values pandas

df.value_counts(normalize=True)