Fix #1724

Provide valid example of Type/Signature.pod6,
X«Constraining signatures of Callables|Callable (constrain)»

Author: tisonkun

doc/Type/Signature.pod6

@@ -286,13 +286,14 @@ a L<Signature> literal right after the parameter (no whitespace allowed):
     f(&g);
     # OUTPUT: «ten10␤»
 
-This shorthand syntax is available only for parameters with the `&` sigil. For
+This shorthand syntax is available only for parameters with the C<&> sigil. For
 others, you need to use the long version:
 
-    sub f($c: where .signature ~~ :(Int, Str))  { say $c(10, 'ten') };
-    sub g(Num $i, Str $s) { $s ~ $i };
-    f(&g);
-    # OUTPUT: «ten10␤»
+    sub f($c where .signature ~~ :(Int, Str))  { say $c(10, 'ten') }
+    sub g(Num $i, Str $s) { $s ~ $i }
+    sub h(Int $i, Str $s) { $s ~ $i }
+    # f(&g); # Constraint type check failed
+    f(&h);   # OUTPUT: «ten10␤»
 
 =head3 X«Constraining Return Types|-->;returns»