Fix a ;; index again (use C<<;;>>)

Author: titsuki

doc/Type/Signature.pod6

@@ -451,7 +451,7 @@ X<|Long Names>
 =head2 Long Names
 
 To exclude certain parameters from being considered in multiple dispatch,
-separate them with a double semi-colon.
+separate them with a double semi-colon X<C<<;;>>|;;>.
 
     multi sub f(Int $i, Str $s;; :$b) { dd $i, $s, $b };
     f(10, 'answer');