Improve example of where-clause on Signature

doc/Type/Signature.pod6

@@ -174,10 +174,10 @@ the C<where>-clause of the last argument has access to all arguments of a
 signature that are not part of a sub-signature. For a sub-signature place
 the C<where>-clause inside the sub-signature.
 
-    sub one-of-them(:$a, :$b, :$c where { $a.defined ^^ $b.defined ^^ $c.defined }) {
-        $a // $b // $c
-    };
-    say one-of-them(c=>42); # OUTPUT: «42␤»
+    sub foo($a, $b where * == $a ** 2) { say "$b is a square of $a" }
+    foo 2, 4; # OUTPUT: «4 is a square of 2␤»»
+    # foo 2, 3;
+    # OUTPUT: «Constraint type check failed in binding to parameter '$b'…»
 
 =head3 Constraining Optional Arguments