fix the versionadded directive markup

Author: xabbuh

reference/forms/types/options/extra_fields_message.rst.inc

@@ -3,7 +3,7 @@
 
 .. versionadded:: 5.1
 
-The support for pluralization was introduced in Symfony 5.1.
+    Pluralization support was introduced in Symfony 5.1.
 
 **type**: ``string`` **default**: ``This form should not contain extra fields.``