use correct attribute name

MatrixPlugin.php

@@ -140,7 +140,7 @@ class MatrixPlugin extends StudIPPlugin implements StandardPlugin
             MatrixClient::get()->inviteIntoRoom(
                 MatrixAccount::requireSystemAccount(),
                 $room->getLinkedRoom(),
-                $matrix->matrix_user_id
+                $matrix->matrix_account_id
             );
         }
     }