rpc: doc: Fix and extend getblockstats examples
rpc: doc: Fix and extend getblockstats examples

This fixes the example curl command for `getblockstats` which is missing
a comma between the params and has single quotes around the second
parameter.

Besides fixing the existing example, this commit adds an additional
example of getting block stats by hash by using a known workaround with
bitcoin-cli to get it to treat the hash parameter as a JSON string by
wrapping it in both single and double quotes.

Co-Authored-By: Andrew Toth <andrewstoth@gmail.com>
Co-Authored-By: Sebastian Falbesoner <sebastian.falbesoner@gmail.com>

Former-commit-id: efa0fbb96ef4db77f796578516221cb85bff4dd7
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin