Update adapter_config.json

Fix error :
Configuration Parsing Warning: In adapter_config.json: "peft.task_type" must be a string

adapter_config.json

@@ -25,5 +25,5 @@
     "up_proj",
     "down_proj"
   ],
-  "task_type": null
+  "task_type": "TEXT_GENERATION"
 }