fix yaml syntax

| Q                | A
| --------------- | ----
| Doc fix?      | yes
| New docs? | no
| Applies to   | all

Author: mhor

cookbook/bundles/installation.rst

@@ -119,15 +119,15 @@ of the bundle's configuration:
 
 The output will look like this:
 
-.. code-block:: text
+.. code-block:: yaml
 
     assetic:
-        debug:                %kernel.debug%
+        debug:                '%kernel.debug%'
         use_controller:
-            enabled:              %kernel.debug%
+            enabled:              '%kernel.debug%'
             profiler:             false
-        read_from:            %kernel.root_dir%/../web
-        write_to:             %assetic.read_from%
+        read_from:            '%kernel.root_dir%/../web'
+        write_to:             '%assetic.read_from%'
         java:                 /usr/bin/java
         node:                 /usr/local/bin/node
         node_paths:           []