help issueasset: clarify syntax for --initial_owner_utxo

[Sjors]

It's unclear from the documentation how --initial_owner_utxo and --issuance_utxo are supposed to be used, and it's different from how JSON is encoded when using bitcoin-cli. So an example would help:

--initial_owner_utxo TXID:VOUT

This code also needs more input validation, as for example the following syntax leads to a crash: --initial_owner_utxo "{txid: <TXID>, vout: <VOUT> }"