Rename `Cheerio#make` to document private status

The confusion in issue #329 made me realize that `Cheerio#make` is a private method that violates the project's naming conventions.

Commit message:

> `Cheerio.prototype.make` is a method intended for internal use only. It
> is attached to the prototype for ease-of-use, as recommended by the
> project's style guide. In order to prevent confusion, this method's name
> should be prefixed with an underscore ("_") character.