RENAME FUNCTION

[paulnegz]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://github.com/PaystackOSS/paystack-python/blob/main/docs/Plan.md#fetch

Expected behavior
fetch should be renamed to get

>>> print(paystack.plan.fetch('PLN_xxxxxxxxxxxxxxx'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'Plan' has no attribute 'fetch'
>>> 
>>> print(paystack.plan.get('PLN_xxxxxxxxxxxxxxx'))
{'status': True, 'message': 'Plan retrieved', 'data': {'subscriptions': [], 'pages': [], 'domain': 'live', 'name': 'Premium Plan', 'plan_code': 'PLN_xxxxxxxxx ...........}}