util: HelpExampleRpc formatting fixup
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 [[https://github.com/bitcoin/bitcoin/pull/18316 | 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

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