Use correct contact key in Protocol\ActivityPub\Transmitter

- Address https://github.com/friendica/friendica/issues/9252#issuecomment-719866843

src/Protocol/ActivityPub/Transmitter.php

@@ -719,7 +719,7 @@ class Transmitter
 				continue;
 			}
 
-			if ($isforum && ($contact['dfrn'] == Protocol::DFRN)) {
+			if ($isforum && ($contact['network'] == Protocol::DFRN)) {
 				continue;
 			}