Fix sonata_block config doc

Missing `~` in config.yml. This causes the following error:

```
  [Symfony\Component\Config\Exception\FileLoaderLoadException]                                                                                                  
  Cannot import resource "/Users/dunglas/workspace/abc/app/config/config.yml" from "/Users/dunglas/workspace/abc/app/config/config_dev.yml".  
                                                                                                                                 
  [Symfony\Component\Yaml\Exception\ParseException]                                                                                           
  Indentation problem in "\/Users\/dunglas\/workspace\/abc\/app\/config\/config.yml" at line 84 (near "sonata.block.service.rss:").  
```