[LazyImage] Fix code block rendering in docs

Author: mttsch

src/LazyImage/doc/index.rst

@@ -111,7 +111,7 @@ It works well for local files, but you may want to customize it to fetch images
 
 To do so you can create a invokable class, the first argument is the filename to fetch:
 
-.. ::
+.. code-block:: php
 
     namespace App\BlurHash;