misc/cgo/errors: update ptr.go comment

Accidentally omitted from submit of CL 53352.

Updates #21306

Change-Id: I022d89c6417fe9371856d49b646eb6294b91657c
Reviewed-on: https://go-review.googlesource.com/53415
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Avelino <t@avelino.xxx>

misc/cgo/errors/ptr.go

@@ -344,7 +344,7 @@ var ptrTests = []ptrTest{
 		fail:    false,
 	},
 	{
-		// Issue #21306.
+		// Test preemption while entering a cgo call. Issue #21306.
 		name:    "preempt-during-call",
 		c:       `void f() {}`,
 		imports: []string{"runtime", "sync"},