Typo in README of confusion_matrix, and suggestion to specify valid values for `normalize`

In the [Output Values](https://github.com/huggingface/evaluate/tree/main/metrics/confusion_matrix#output-values) section are some typos:

"""
Minimum possible value is 0. Maximum possible value is 1.0,
or the number of examples input, if `normalize` is set to `True`.
"""

The corrections are in bold:

"""
Minimum possible value is 0. Maximum possible value is 1.0,
or the number of examples **in the** input, if `normalize` is set to **`None` (default)**.
"""

Additionally, I would specify in the [Inputs](https://github.com/huggingface/evaluate/tree/main/metrics/confusion_matrix#inputs) section the permitted values for `normalize`: {'pred', 'all', 'true'} or `None`.