fix YAML code examples (#428)

fix YAML code example

Author: xabbuh

Diff for: docs/custom_provider.rst

@@ -82,7 +82,7 @@ Then, configure the services linked to this new provider.
       app.menu_provider:
             class: AppBundle\Provider\CustomMenuProvider
             arguments:
-              - @knp_menu.factory
+              - '@knp_menu.factory'
             tags:
               - { name: knp_menu.provider }
 

Diff for: docs/custom_renderer.rst

@@ -29,9 +29,9 @@ The configuration is then the following:
             # The class implements Knp\Menu\Renderer\RendererInterface
             class: AppBundle\Menu\CustomRenderer
             arguments:
-                - @knp_menu.matcher
-                - "%knp_menu.renderer.list.options%"
-                - "%kernel.charset%"
+                - '@knp_menu.matcher'
+                - '%knp_menu.renderer.list.options%'
+                - '%kernel.charset%'
                 # add your own dependencies here
             tags:
                 # The alias is what is used to retrieve the menu