[Form] Fix a minor syntax issue

Author: javiereguiluz

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

@@ -4,7 +4,7 @@
 **type**: ``string`` or ``TranslatableMessage`` **default**: The label is "guessed" from the field name
 
 Sets the label that will be used when rendering the field. Setting to ``false``
-will suppress the label.
+will suppress the label::
 
     use Symfony\Component\Translation\TranslatableMessage;