Correct the documentation for the slugify function

The previous requirements are incorrect, the method needs to accept two parameters.

I discovered this while looking into: https://github.com/tomchristie/mkdocs/issues/212

You can see the requirement happen here as two arguments are passed. https://github.com/d0ugal/Python-Markdown/blob/2.5.2-final/markdown/extensions/toc.py#L178