util: HelpExampleRpc formatting fixup

Summary:
to conform to json-rpc spec as per
https://www.jsonrpc.org/specification#examples

This is a backport of Core PR18316

Test Plan:
Look at the curl example in the help message of any rpc command
src/bitcoin-cli help getchaintips

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8831