Rework Parameter.type_captures example

Author: stoned

doc/Type/Parameter.pod6

@@ -342,12 +342,19 @@ given $! { .message.say };
 Type captures may be used at the same time as
 L<type constraints|/type/Signature#Type_constraints>.
 
-=for code
+=begin code
 sub d(::T Numeric $x, T $y) {};
 d(4, 5);            # OK
 d(4e0, 5e0);        # OK
-d(4e0, 5);          # Fails when binding $y
-d("four", "five");  # Fails when binding $x
+
+try d(4e0, 5);
+given $! { .message.say };
+# OUTPUT: «Type check failed in binding to parameter '$y'; expected Num but got Int (5)␤»
+
+try d("four", "five");
+given $! { .message.say };
+# OUTPUT: «Type check failed in binding to parameter '$x'; expected Numeric but got Str ("four")␤»
+=end code
 
 =head2 method sub_signature