Fix TOC URLs

The `postprocessor()` method is changing TOC URLs and including `file://` as a prefix. I think it's enough to the TOC links to be just as `"#something"`. So, it looks like there's no need for the whole path prefixing it as in `"file:///home/user/file.html#something"`. It would help everyone who is generating HTML in st2 and then pasting it to some web-based editor.