Fix default default for optional params

Blocks actually default to Mu not Any

Author: zoffixznet

doc/Type/Signature.pod6

@@ -278,10 +278,11 @@ objects and discovering them with the C<.DEFINITE> method.
 
 Keep in mind all parameters have values; even optional ones have
 default defaults that are the type object of the constrained type
-for explicit type constraints or an L<Any> type object if no
-explicit type constraint exists. This means that if you use the 
-C<:D> type smiley, you'd need to provide a default value or make
-the parameter required. Otherwise, the default default would be
+for explicit type constraints. If no explicit type constraint exists,
+the default default is an L<Any> type object for methods, submethods,
+and subroutines, and a L<Mu> type object for blocks. This means that
+if you use the  C<:D> type smiley, you'd need to provide a default
+value or make the parameter required. Otherwise, the default default would be
 a type object, which would fail the definiteness constraint.
 
     sub divide (Int:D :$a = 2, Int:D :$b!) { say $a/$b }