Replace code-block php with shorthand

Author: SzymonKaminski

service_container/alias_private.rst

@@ -177,9 +177,7 @@ This means that when using the container directly, you can access the
 
     When using ``#[AsAlias]`` attribute, you may omit passing ``id`` argument
     if the class implements exactly one interface. ``MailerInterface`` will be
-    alias of ``PhpMailer``:
-
-    .. code-block:: php
+    alias of ``PhpMailer``::
 
         // src/Mail/PhpMailer.php
         namespace App\Mail;