[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 PR12951

Test Plan:

just read it

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

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