testing: remove redundant variable capture from subtest example
Loop variable capturing hasn't been needed like this since Go 1.21;
remove it from the example.
Change-Id: I231dcfdb57832e32d524f156a605ba36d1c9d6d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/660176
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>