[doc] Fix comment in FindForkInGlobalIndex
[doc] Fix comment in FindForkInGlobalIndex

The comment erroneously implies that we're searching `chainActive` for the
first block common to `locator`, but we're using the parameter `chain`.

DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin,peercoin/peercoin,peercoin/peercoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin