Update containers.pod6

Fixing the code example about custom containers. Without the "-rw" the code dies.

Author: holli-holzer

doc/Language/containers.pod6

@@ -400,7 +400,7 @@ work with types in Perl 6.
 
     sub lucky(::T $type) {
         my T $c-value; # closure variable
-        return Proxy.new(
+        return-rw Proxy.new(
             FETCH => method () { $c-value },
             STORE => method (T $new-value) {
                 X::OutOfRange.new(what => 'number', got => '13', range => '-∞..12, 14..∞').throw