MinimumRequirements: fix macOS 26 typo

I think this actually intended to say macOS 15 at the time, but now we
have macOS 26 builders!

Change-Id: I6a6a636cacea06ac991ccc4b81ab8a77b775225a
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/755100
Auto-Submit: Michael Pratt <mpratt@google.com>
Commit-Queue: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>