convert amounts to XEC in RPC examples

Summary: RPC examples still used BCHA as a unit.

Test Plan:
ninja

Print the help for a couple of command to check the output visually.
src/bitcoin-cli help fundrawtransaction

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10266