That looks like a really old version of turndown.js, pre-ES6. The link to turndown in the readme is the old one, too. How long ago did you make this? Anyway, you need to put turndown's license in the js file or someplace if you're not going to use npm for some reason.
> The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
I would just put it in a comment at the top of turndown.js. That's enough to fulfil the license requirements. I know a lot of people don't care, but good to do it right. Definitely don't replace your license with theirs though :)