update config.json

update max_position_embeddings as 4096, which is used in `run_summarization.py` in the huggingface library.

config.json

@@ -69,7 +69,7 @@
   },
   "max_decoder_position_embeddings": 1024,
   "max_encoder_position_embeddings": 4096,
-  "max_position_embeddings": 1024,
+  "max_position_embeddings": 4096,
   "model_type": "led",
   "normalize_before": false,
   "normalize_embedding": true,