[backport#12951] [doc] Fix comment in FindForkInGlobalIndex
[backport#12951] [doc] Fix comment in FindForkInGlobalIndex

Summary:
[doc] Fix comment in FindForkInGlobalIndex (James O'Beirne)

Pull request description:

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

---

Backport of Core [[https://github.com/bitcoin/bitcoin/pull/12951 | PR12951]]

Test Plan:
  just read it

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7622

Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc