Merge bitcoindevkit/bdk#2066: Correct rpc example command syntax

d6ba0ae458485d8725a1dc1a4410cd6bb75da2ef Correct rpc example command syntax (Adam Gibson)

Pull request description:

  Several of the steps in the README for example_bitcoind_rpc_polling don't work as written. The most important was at the start, there is a missing "init" command that otherwise gives a file error. Also the specification of regtest network needed is a bit different than what is written.

  Apologies, didn't bother with template, this is just a testing writeup fix (whether correct or not).

ACKs for top commit:
  ValuedMammal:
    ACK d6ba0ae458485d8725a1dc1a4410cd6bb75da2ef

Tree-SHA512: b287edea7868da12be7ea06fd5352b168f4ab6f389c4b450e77933161b2c89dc4bfad721ca2e68bf21f8f39151bc5f12e716c88dacb287015464b38256fbdc52