Adds example for is default on attributes

The item in #2632 was already covered, but this add an illustrative example.

Author: JJ

doc/Type/Attribute.pod6

@@ -30,6 +30,12 @@ argument of C<is default> will set the default item value or hash value.
     $c.a = Nil;
     say $c;
     # OUTPUT: «C.new(a => 666)␤C.new(a => 42)␤»
+    class Foo {
+        has @.bar is default(42) is rw
+    };
+    my $foo = Foo.new( bar => <a b c> );
+    $foo.bar =Nil;
+    say $foo; # OUTPUT: «Foo.new(bar => [42])␤»
 
 =head2 X<Trait is required|trait,is required (Attribute)>