Chainparams.cpp needs updating for clarity of halving strategy

[bigdaveakers]

BTCP halving strategy is currently 210,000 blocks but the code is not clear and still suggests that it is 840,000.

There is a bit shift that reduces this to 210,000 but commentary is not clear. The rebase code appears to address this by setting nSubsidyHalvingInterval directly to 210,000

[mattpass]

This is the code mentioned above:
https://github.com/BTCPrivate/BitcoinPrivate/blob/6c8ed84c5185fa522b57e08d1494bab2020da39a/src/main.cpp#L1440

Always a good idea to point code out with the URL and line number(s) where possible 😀