[docs]: Contract interaction tutorial is missing the `mint` step before private transfer

Page: https://docs.aztec.network/tutorials/codealong/simple_dapp/contract_interaction

missing [this](https://github.com/AztecProtocol/aztec-packages/blob/4dd51eb0a315687fc701b1da0458b71a99ab68a7/yarn-project/end-to-end/src/sample-dapp/index.mjs#L29) function