Rework Parameter.type_captures example

Author: stoned

doc/Type/Parameter.pod6

@@ -326,11 +326,18 @@ The type used may change from call to call.  Once they are defined,
 type captures can be used wherever you would use a type, even later
 in the same signature:
 
-=for code
+=begin code
 sub c(::T $x, T $y, $z) { my T $zz = $z };
 c(4, 5, 6);          # OK
-c(4, 5, "six");      # Fails when assigning to $zz, wants Int not Str
-c("four", 5, "six"); # Fails when binding $y, wants Str, not Int
+
+try c(4, 5, "six");
+given $! { .message.say };
+# OUTPUT: «Type check failed in assignment to $zz; expected Int but got Str ("six")␤»
+
+try c("four", 5, "six");
+given $! { .message.say };
+# OUTPUT: «Type check failed in binding to parameter '$y'; expected Str but got Int (5)␤»
+=end code
 
 Type captures may be used at the same time as
 L<type constraints|/type/Signature#Type_constraints>.