website: remove unused root path on building website (#3248)

We no longer use the `--root` option of `build_website.py` since we have
moved to jqlang.org (from jqlang.github.io/jq where the root was `/jq`).
We don't need the option on local development either since `http.server`
has the `-d` option to specify the directory to serve from.

docs/content/download/default.yml

@@ -239,9 +239,9 @@ body:
       #### Building the documentation
 
       jq's documentation is compiled into static HTML using Python.
-      To build the docs, run `pipenv run python3 build_website.py --root /output`
+      To build the docs, run `pipenv run python3 build_website.py`
       in the `docs/` directory. To serve them locally, you can run
-      `python3 -m http.server`. You'll need a few Python dependencies,
+      `python3 -m http.server -d output`. You'll need a few Python dependencies,
       which can be installed by following the instructions in `docs/README.md`.
 
       The man page is built by `make jq.1`, or just `make`, also from