broken link in erc20 readme

[kirilligum]

in https://github.com/cartesi/rollups-examples/blob/main/erc20/README.md , the link in

A deposit must have a payload, whose format is defined at the [ERC-20 Portal contract](https://github.com/cartesi/rollups/blob/main/onchain/rollups/contracts/portals/IERC20Portal.sol), which is part of the Cartesi Rollups contract.

is broken.

should probably be onchain/rollups/contracts/portals/ERC20Portal.sol