Fix package.json.

Author: surfnerd

Project/Packages/manifest.json

@@ -3,7 +3,7 @@
     "com.unity.ads": "2.0.8",
     "com.unity.analytics": "3.2.3",
     "com.unity.collab-proxy": "1.2.15",
-    "com.unity.ml-agents": "0.16.0-preview",
+    "com.unity.ml-agents": "file:../../com.unity.ml-agents",
     "com.unity.package-manager-ui": "2.0.8",
     "com.unity.package-validation-suite": "0.7.15-preview",
     "com.unity.purchasing": "2.0.3",