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>

src/testing/testing.go

@@ -345,7 +345,6 @@
 //
 //	func TestGroupedParallel(t *testing.T) {
 //	    for _, tc := range tests {
-//	        tc := tc // capture range variable
 //	        t.Run(tc.Name, func(t *testing.T) {
 //	            t.Parallel()
 //	            ...