Fix examples for File System Provider API.

chrome/common/extensions/docs/examples/api/fileSystemProvider/archive/manifest.json

 {
   "name": "Fake Archive Handler App",
   "version": "0.1",
   "manifest_version": 2,
   "description": "Demonstrate File System Provider API usage for apps.",
   "permissions": [
     "fileSystemProvider",
     {"fileSystem": ["retainEntries"]},
     "storage"
   ],
   "file_handlers": {
     "fake": {
       "types": ["application/fake"],
       "extensions": ["fake"],
       "title": "Open fake archive"
     }
   },
+  "file_system_provider_capabilities": {
+    "multiple_mounts": true,
+    "source": "file"
+  },
   "app": {
     "background": {
       "scripts": [
         "background.js"
       ]
     }
   }
 }