Minor fixes in the documentation for Plug.Debugger

There was a Markdown error (PLUG\_EDITOR was written `PLUG_EDITOR` and not `PLUG\_EDITOR`) which made an entire section of the docs italic. Starting from that, I fixed minor stuff in the docs and renamed a couple of variables (see the commented lines in the diff).

I also took advantage of `|>` in `get_editor/3` since that's the perfect `|>` use case :).