Update package description. (#3906)

com.unity.ml-agents/package.json

   "displayName": "ML Agents",
   "version": "1.0.0-preview",
   "unity": "2018.4",
-  "description": "Add interactivity to your game with Machine Learning Agents trained using Deep Reinforcement Learning.",
+  "description": "Use state-of-the-art machine learning to create intelligent character behaviors in any Unity environment (games, robotics, film, etc.).",
   "dependencies": {
     "com.unity.barracuda": "0.7.0-preview"
   }