Improve the example

To reflect what is indicated by @lizmat here
https://stackoverflow.com/questions/41531393/can-i-use-a-standalone-signature-as-a-signature-in-perl-6#comment84619436_41543023
and also reflect the intent of putting signatures into variables, as
said in #1748 (by myself, but still). Also modifies the example
changed in #1777, so thanks to @jimav for drawing this to our attention.

Author: JJ

doc/Type/Signature.pod6

@@ -58,6 +58,9 @@ Smart matching signatures against a List is supported.
     my $sig = :(Int $i, Str $s);
     say (10, 'answer') ~~ $sig;
     # OUTPUT: «True␤»
+    my $ßub = sub ( Str $ß, Int $þ ) { return $ß xx $þ };
+    say $ßub.signature ~~ :( Str, Int );
+    # OUTPUT: «True␤»
     given $sig {
         when :(Str, Int) { say 'mismatch' }
         when :($, $)     { say 'match' }