Add separate code-block for blocks (#360)

Resources/doc/index.rst

@@ -152,7 +152,9 @@ Sharing Objects between Fixtures
 When using multiple fixtures files, you can reuse PHP objects across different
 files thanks to the **object references**. Use the ``addReference()`` method to
 give a name to any object and then, use the ``getReference()`` method to get the
-exact same object via its name::
+exact same object via its name:
+
+.. code-block:: php
 
     // src/DataFixtures/UserFixtures.php
     // ...
@@ -171,6 +173,8 @@ exact same object via its name::
         }
     }
 
+.. code-block:: php
+
     // src/DataFixtures/GroupFixtures.php
     // ...
     class GroupFixtures extends Fixture