Incorrect sample code in README

[Atheuz]

Hi,

In the README, here: https://github.com/vcoder4c/cron-validator/blob/master/README.md#sample

There is this bit of sample code for validating a cron expression:

from cron_validator import CronValidator

assert CronValidator.parse('* * * * *') is not None # valid
assert CronValidator.parse('*/3 * * * *') is not None # valid
assert CronValidator.parse('*/61 * * * *') is None # invalid

This is not what it actually does:

CronValidator.parse('* * * * *')  is not None  => True
CronValidator.parse('*/3 * * * *') is not None => True
CronValidator.parse('*/61 * * * *') => ValueError: Invalid expression part 0

I.E. passing an invalid value to CronValidator.parse makes it raise a ValueError, it doesn't return None.

I don't mind the ValueError, I just didn't understand why the described behaviour and actual behaviour were different.

Also:

CronValidator.parse(0) => AttributeError: 'int' object has no attribute 'split'  # Should be a ValueError?