BIP-322: fix proof-of-funds inputs wording

[kurahin]

The Signing section previously stated that signers may add additional outputs to to_sign for proving control of funds. This contradicts the Full (Proof of Funds) subsection, which specifies that the claimed UTXOs must be included as additional inputs of to_sign and validated against the current UTXO set, and also conflicts with the requirement that to_sign has exactly one output (an OP_RETURN).