Fixed wrong indentation in documentation

Fixes an error:

```
Running Sphinx v1.2.2
loading pickled environment... done
building [html]: targets for 1 source files that are out of date
updating environment: 0 added, 1 changed, 0 removed
reading sources... [100%] cookbook/recipe_sortable_listing                                                                                                                                                                                    

Warning, treated as error:
/.../SonataAdminBundle/Resources/doc/cookbook/recipe_sortable_listing.rst:26: WARNING: Bullet list ends without a blank line; unexpected unindent.

make: *** [test] Error 1
```