Update config.json

Related: https://github.com/huggingface/transformers/issues/22903

config.json

@@ -41,7 +41,7 @@
     },
     "initializer_factor": 1.0,
     "initializer_range": 0.02,
-    "is_decoder": false,
+    "is_decoder": true,
     "is_encoder_decoder": false,
     "label2id": {
       "LABEL_0": 0,