Typo in BlockFrost backend

[sassythesasquatchh]

The string method should be in all lowercase. Currently, this bug causes a critical error in my application.
https://github.com/Python-Cardano/pycardano/blob/db56cbe0d093f98c30dc5ae48f57a30c96374268/pycardano/backend/blockfrost.py#L177

[cffls]

This issue is fixed. Feel free to reopen otherwise.