Fix typo in the queue test.

docs/primer.md

@@ -274,8 +274,8 @@ First, define a fixture class. By convention, you should give it the name
 class QueueTest : public ::testing::Test {
  protected:
   void SetUp() override {
-     q1_.Enqueue(1);
-     q2_.Enqueue(2);
+     q0_.Enqueue(1);
+     q1_.Enqueue(2);
      q2_.Enqueue(3);
   }