Fix testable example: ExampleObjectPool

Output is only checked when it is the last comment block in the testable example. See this golang issue for more info: golang/go#48362

src/x/pool/example_test.go

@@ -50,12 +50,13 @@ func ExampleObjectPool() {
 	o := p.Get().(*exampleObject)
 
 	fmt.Printf("Retrieved struct should have default values: %+v", o)
-	// Output: Retrieved struct should have default values: &{a:0 b:0 c:0}
 
 	// Use the exampleObject.
 	_ = o
 
 	// Reset the exampleObject and return it to the pool.
 	o.reset()
 	p.Put(o)
+
+	// Output: Retrieved struct should have default values: &{a:0 b:0 c:0}
 }