changed is_erc20_transfer to use Transfer signature instead of Approval

[ChadRosseau]

Motivation

fixes #231

Solution

Changed erc20_transfers.rs line 104 from:

log.topics()[0] == ERC20::Approval::SIGNATURE_HASH

to

log.topics()[0] == ERC20::Transfer::SIGNATURE_HASH

PR Checklist

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