Modify example in the primer to match Testing FAQ.

The CtorVsSetUp section of the FAQ says that constructors and destructors should be preferred over SetUp() and TearDown(), because they will automatically chain up to the fixture's base class, whereas for methods the user must remember to add the chaining manually.

PiperOrigin-RevId: 624273474
Change-Id: Ida41aae193d417eaf996587c7ae1a0099a8cab32

docs/primer.md

@@ -273,23 +273,24 @@ First, define a fixture class. By convention, you should give it the name
 ```c++
 class QueueTest : public testing::Test {
  protected:
-  void SetUp() override {
+  QueueTest() {
      // q0_ remains empty
      q1_.Enqueue(1);
      q2_.Enqueue(2);
      q2_.Enqueue(3);
   }
 
-  // void TearDown() override {}
+  // ~QueueTest() override = default;
 
   Queue<int> q0_;
   Queue<int> q1_;
   Queue<int> q2_;
 };
 ```
 
-In this case, `TearDown()` is not needed since we don't have to clean up after
-each test, other than what's already done by the destructor.
+In this case, we don't need to define a destructor or a `TearDown()` method,
+because the implicit destructor generated by the compiler will perform all of
+the necessary cleanup.
 
 Now we'll write tests using `TEST_F()` and this fixture.
 
@@ -326,11 +327,9 @@ would lead to a segfault when `n` is `NULL`.
 When these tests run, the following happens:
 
 1.  GoogleTest constructs a `QueueTest` object (let's call it `t1`).
-2.  `t1.SetUp()` initializes `t1`.
-3.  The first test (`IsEmptyInitially`) runs on `t1`.
-4.  `t1.TearDown()` cleans up after the test finishes.
-5.  `t1` is destructed.
-6.  The above steps are repeated on another `QueueTest` object, this time
+2.  The first test (`IsEmptyInitially`) runs on `t1`.
+3.  `t1` is destructed.
+4.  The above steps are repeated on another `QueueTest` object, this time
     running the `DequeueWorks` test.
 
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