Fix bad example

Part of #1438

Author: zoffixznet

doc/Language/subscripts.pod6

@@ -678,7 +678,7 @@ to that attribute (using a custom type constraint to make sure users don't
 insert anything invalid into it):
 
     =begin code :skip-test
-    class HTTP::Header does Associative is Iterable {
+    class HTTP::Header does Associative does Iterable {
         subset StrOrArrayOfStr where Str | ( Array & {.all ~~ Str} );
 
         has %!fields of StrOrArrayOfStr