Fix code blocks

Author: dunglas

components/serializer.rst

@@ -149,7 +149,7 @@ Attributes Groups
 Sometimes, you want to serialize different set of attributes from your
 entities. Groups are a handy way to achieve this need.
 
-Lets start with a simple Plain Old PHP object.
+Lets start with a simple Plain Old PHP object::
 
     namespace Acme;
 
@@ -164,21 +164,21 @@ and YAML. The :class:Symfony\Component\Serializer\Mapping\Factory\ClassMetadataF
 that will be used by the normalizer must be aware of the format to use.
 
 If you want to use annotations to define groups, initialize the :class:Symfony\Component\Serializer\Mapping\Factory\ClassMetadataFactory
-like the following:
+like the following::
 
     use Symfony\Component\Serializer\Mapping\Loader\AnnotationLoader;
     use Symfony\Component\Serializer\Mapping\Factory\ClassMetadataFactory;
 
     $classMetadataFactory = new ClassMetadataFactory(new AnnotationLoader(new AnnotationReader()));
 
-If you prefer XML:
+If you prefer XML::
 
     use Symfony\Component\Serializer\Mapping\Factory\ClassMetadataFactory;
     use Symfony\Component\Serializer\Mapping\Loader\XmlFileLoader;
 
     $classMetadataFactory = new ClassMetadataFactory(new XmlFileLoader('/path/to/your/definition.xml'));
 
-And for YAML:
+And for YAML::
 
     use Symfony\Component\Serializer\Mapping\Factory\ClassMetadataFactory;
     use Symfony\Component\Serializer\Mapping\Loader\YamlFileLoader;
@@ -234,7 +234,7 @@ Then, create your groups definition:
             </class>
         </serializer>
 
-You are now able to serialize only attributes in the groups you want:
+You are now able to serialize only attributes in the groups you want::
 
     use Symfony\Component\Serializer\Serializer;
     use Symfony\Component\Serializer\Normalizer\GetSetMethodNormalizer;