[docs] use CashAddr throughout sendmany help

Summary:
D8164 fixed a wrong help example in sendmany and I took the opportunity
to change the example addresses to CashAddr testnet ones, this changes
the other examples for consistency lest I forget to do it later

Test Plan:

ninja
bitcoin-cli help sendmany

read it

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8165