file extension should be knd.gz in example README

[fengggli]

• snd.gz should be knd.gz

• curl -s https://github.com/attractivechaos/kann/releases/download/v0/kann-data.tgz | tar -zxf - failed here (ubuntu 16.04)

  ~: curl -s https://github.com/attractivechaos/kann/releases/download/v0/kann-data.tgz | tar -zxf - 
  gzip: stdin: not in gzip format
  tar: Child returned status 1
  tar: Error is not recoverable: exiting now

  So I added a note in README to just use wget instead