Fix configuration field names in mlc-chat-config.json

- Corrected "field_model_type" to "model_type"
- Corrected "field_model_config" to "model_config"

mlc-chat-config.json

@@ -1,8 +1,8 @@
 {
   "version": "0.1.0",
-  "field_model_type": "mistral",
+  "model_type": "mistral",
   "quantization": "q4f32_1",
-  "field_model_config": {
+  "model_config": {
     "hidden_size": 4096,
     "intermediate_size": 14336,
     "num_attention_heads": 32,
@@ -78,4 +78,4 @@
   "mean_gen_len": 128,
   "max_gen_len": 512,
   "shift_fill_factor": 0.3
-}
+}