Renamed method of templating helper

I wanted to submit some changes to the CMF docs to add the new PHP templating helpers, and I discovered that it would be much nicer to use `render` instead of `renderBlock`.

Before
````php
<?php echo $view['sonata_block']->renderBlock(...) ?>
````

After
````php
<?php echo $view['sonata_block']->render(...) ?>
````

This'll also play nice with the CmfBlockBundle:

Before
````php
<?php echo $view['blocks']->renderBlock(...) ?>
````

After
````php
<?php echo $view['blocks']->render(...) ?>
````

I'm sorry I didn't discovered this when submiting the previous PR.