message: docs: update nydusd curl example
  We have changed the API but forgot to update the example.
diff: @@ -217,9 +217,15 @@ Then use curl to mount a bootstrap to `/path/to/mountpoint/sub`:
  curl --unix-socket api.sock \
  -X POST "http://localhost/api/v1/mount?mountpoint=/sub" \
  -H "Content-Type: application/json" \
  - -d '{"source":"/path/to/bootstrap","config":"/path/to/config.json"}'
  + -d '{
  + "source":"/path/to/bootstrap",
  + "fs_type":"rafs",
  + "config":"{\"device\":{\"backend\":{\"type\":\"localfs\",\"config\":{\"dir\":\"blobs\"}},\"cache\":{\"type\":\"blobcache\",\"config\":{\"work_dir\":\"cache\"}}},\"mode\":\"direct\",\"digest_validate\":true}"
  + }'
  ```
  +The `config` field is a JSON format string that can be obtained by `cat rafs.config | jq tostring`.
  +
  ### Multiple Pseudo Mounts
  One single nydusd can have multiple pseudo mounts within a mountpoint.
  