Fix math issue in open-access section

[smartcontracts]

Noticed this bug, bytes have 256 possible values not 16. Nibbles have 16 possible values. Either need to change the language to refer to nibbles or update the math. I personally think updating the math is cleaner, more people know about bytes than nibbles: https://github.com/smartcontracts/eth2-book/blob/467e1393fe861cb956e4db785532e8bdb66a92d5/src/book/02__blockchains/02__open-access.md?plain=1#L45