Fix documentation for FSP API.

The capabilities section is a map, not an array. This CL fixes the example
snippet.

TEST=Testing docs server.
BUG=489217

Review URL: https://codereview.chromium.org/1130943006

Cr-Commit-Position: refs/heads/master@{#330542}

chrome/common/extensions/docs/templates/intros/fileSystemProvider.html

@@ -11,11 +11,11 @@ For example:</p>
     "fileSystemProvider"
   ],
   ...
-  "file_system_provider_capabilities": [
+  "file_system_provider_capabilities": {
     "configurable": true,
     "multiple_mounts": true,
     <span id="manifest-source">"source": "network"</span>
-  ],
+  },
   ...
 }
 </pre>