Add model_type to config.json

`AutoModelForMaskedLM` fails to infer the model type from the name of the model, and cannot load this model if the `model_type` property is missing from the config file.

I assumed the correct value to be "bert" from the name of the model, apologies if it is the wrong type for this model.

config.json

@@ -6,6 +6,7 @@
   "initializer_range": 0.02,
   "intermediate_size": 3072,
   "max_position_embeddings": 512,
+  "model_type": "bert",
   "num_attention_heads": 12,
   "num_hidden_layers": 12,
   "type_vocab_size": 2,