Corrected contract address colname from erc20 to erc721 for erc721_transfers

[ChadRosseau]

Motivation

Fixes #230

Incorrect contract address column naming for erc721_transfers.

Solution

Adjusted variable naming and column name string for erc721_transfers contract addresses from erc20 to erc721.

PR Checklist

• [ ] Added Tests
• [ ] Added Documentation
• [X] Breaking changes

May break existing applications which are already using incorrect colname erc20 for erc721_transfers.