[Docs] Correct `sendmany` curl example #12077

The example curl command in the sendmany RPC help results in an error:

{"result":null,"error":{"code":-1,"message":"JSON value is not an object as expected"},"id":"curltest"}

because - as reported in #12067 - the second element in the params array should be a JSON object.

This PR removes the escaped backslash and quote characters that serialize the JSON object to a string and make it a plain JSON object:

curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "sendmany", "params": ["", {"1D1ZrZNe3JUo7ZycKEYQQiQAWd9y54F4XX":0.01,"1353tsE8YMTA4EuV7dgUXGjNFf9KpVvKHz":0.02}, 6, "testing"] }' -H 'content-type: text/plain;' http://127.0.0.1:8332/

which (when executed with valid addresses) yields the expected result:

{"result":"c446213f06ee4489660ed5dc47d65daca27345d0b5fc5bd344ebca78b2d87356","error":null,"id":"curltest"}

Concept ACK, thanks for helping out :)

It'd be better to move the "Fixes #12067" text into the body of the PR not the title or the commit message though

tested ACK 0d663f8, looks good to me, IMO the commit message is clear enough.