minor #13981 Fixed the Quick Tour sample code to work just by copy and paste. (okazy)

This PR was submitted for the master branch but it was merged into the 4.4 branch instead.

Discussion
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Fixed the Quick Tour sample code to work just by copy and paste.

Beginners try to imitate the code of the quick tour as it is, but it does not work as it is because there is no `<?php`. Those pages are for beginners, so it's better to work just by copy and paste.

Commits
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8f3ff01 Fixed the quick tour sample code to work just by copy and paste.

Author: javiereguiluz

quick_tour/flex_recipes.rst

@@ -75,6 +75,7 @@ Thanks to Flex, after one command, you can start using Twig immediately:
 
 .. code-block:: diff
 
+    <?php
     // src/Controller/DefaultController.php
     namespace App\Controller;
 
@@ -153,6 +154,7 @@ Rich API Support
 
 Are you building an API? You can already return JSON from any controller::
 
+    <?php
     // src/Controller/DefaultController.php
     namespace App\Controller;
 
@@ -189,6 +191,7 @@ But like usual, we can immediately start using the new library. Want to create a
 rich API for a ``product`` table? Create a ``Product`` entity and give it the
 ``@ApiResource()`` annotation::
 
+    <?php
     // src/Entity/Product.php
     namespace App\Entity;
 

quick_tour/the_architecture.rst

@@ -21,6 +21,7 @@ Want a logging system? No problem:
 This installs and configures (via a recipe) the powerful `Monolog`_ library. To
 use the logger in a controller, add a new argument type-hinted with ``LoggerInterface``::
 
+    <?php
     // src/Controller/DefaultController.php
     namespace App\Controller;
 
@@ -89,6 +90,7 @@ To keep your code organized, you can even create your own services! Suppose you
 want to generate a random greeting (e.g. "Hello", "Yo", etc). Instead of putting
 this code directly in your controller, create a new class::
 
+    <?php
     // src/GreetingGenerator.php
     namespace App;
 
@@ -105,6 +107,7 @@ this code directly in your controller, create a new class::
 
 Great! You can use this immediately in your controller::
 
+    <?php
     // src/Controller/DefaultController.php
     namespace App\Controller;
 
@@ -135,6 +138,7 @@ difference is that it's done in the constructor:
 
 .. code-block:: diff
 
+    <?php
     // src/GreetingGenerator.php
     + use Psr\Log\LoggerInterface;
 
@@ -167,6 +171,7 @@ by creating an event subscriber or a security voter for complex authorization
 rules. Let's add a new filter to Twig called ``greet``. How? Create a class
 that extends ``AbstractExtension``::
 
+    <?php
     // src/Twig/GreetExtension.php
     namespace App\Twig;
 

quick_tour/the_big_picture.rst

@@ -80,6 +80,7 @@ doesn't exist yet, so let's create it!
 In ``src/Controller``, create a new ``DefaultController`` class and an ``index``
 method inside::
 
+    <?php
     // src/Controller/DefaultController.php
     namespace App\Controller;
 
@@ -115,6 +116,7 @@ like a wildcard that matches anything. And it gets better! Update the controller
 
 .. code-block:: diff
 
+    <?php
     // src/Controller/DefaultController.php
     namespace App\Controller;
 
@@ -153,6 +155,7 @@ Instead, add the route *right above* the controller method:
 
 .. code-block:: diff
 
+    <?php
     // src/Controller/DefaultController.php
     namespace App\Controller;
 
@@ -173,6 +176,7 @@ This works just like before! But by using annotations, the route and controller
 live right next to each other. Need another page? Add another route and method
 in ``DefaultController``::
 
+    <?php
     // src/Controller/DefaultController.php
     namespace App\Controller;