How do I erase a disk on my refurbished MacBook Air 2013?

I bought my 2013 MacBook Air refurbished. I am now trying to sell it, but I want to wipe the contents first. I followed the Apple How To page, but my laptop does not have Macintosh HD or Macintosh HD - Data under the Internal Disk menu. My data is stored alone in a disk called "Untitled". Is it ok to erase this if there is not another disk on my laptop?