erc20_transfers collect_by_transaction uses Approval signature instead of Transfer signature

[ChadRosseau]

Description
In erc20_transfers.rs, the is_erc20_transfer function on line 101 is as follows:

fn is_erc20_transfer(log: &Log) -> bool {
    log.topics().len() == 3 &&
        log.data().data.len() == 32 &&
        log.topics()[0] == ERC20::Approval::SIGNATURE_HASH
}

Note that it checks for an Approval log signature hash instead of a Transfer signature hash. Since this function is used in CollectByTransaction, it will incorrectly collect Approval logs instead of Transfer logs.

Line 104 should instead be as follows, which mirrors the check on line 62 for CollectByBlock:

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