Wrong examples in docstrings

[dk-WZFinTech]

There are wrong examples on description for methods get_rates and get_rate

python-exchangeratesapi/exchangeratesapi/api.py

Line 92 in 86eed44

>>> api.get_rate('USD', 'EUR', date="2019-09-12")

should be:

• api.get_rates('USD', ['EUR'], start_date="2019-09-12") for get_rates

• api.get_rate('USD', 'EUR', start_date="2019-09-12") for get_rate