Made the 'is default' a sub-heading in the Sub type

Author: nicqrocks

doc/Type/Sub.pod6

@@ -61,7 +61,7 @@ to it (or even themselves) and we can apply traits to objects at runtime.
     &trait_mod:<is>(&bar, :foo);
     # OUTPUT«is foo called␤is foo called␤»
 
-=head trait is default (Sub class)
+=head2 trait is default (Sub class)
 
 There is a special trait for C<Sub>s called C<is default>. This trait is
 designed as a way to disambiguate C<multi> function calls that would normally