message: Doc: fix mount API usage
  Should put `mountpoint` parameter to query string of HTTP request.
diff: @@ -213,9 +213,9 @@ Then use curl to mount a bootstrap to `/path/to/mountpoint/sub`:
  ``` shell
  curl --unix-socket api.sock \
  - -X POST "http://localhost/api/v1/mount" \
  + -X POST "http://localhost/api/v1/mount?mountpoint=/sub" \
  -H "Content-Type: application/json" \
  - -d '{"source":"<path-to-bootstrap>","fstype":"rafs","mountpoint":"/sub","config":"/path/to/config.json"}'
  + -d '{"source":"/path/to/bootstrap","config":"/path/to/config.json"}'
  ```
  ### Multiple Pseudo Mounts
  