Comment that q0_ in primer should remain empty

PiperOrigin-RevId: 496642643
Change-Id: I178efeda76cbf46e5f4bb91454c205b9a060a4ea

docs/primer.md

@@ -274,6 +274,7 @@ First, define a fixture class. By convention, you should give it the name
 class QueueTest : public ::testing::Test {
  protected:
   void SetUp() override {
+     // q0_ remains empty
      q1_.Enqueue(1);
      q2_.Enqueue(2);
      q2_.Enqueue(3);