Fix broken "make livehtml" command

The documentation suggests running `make livehtml` to automatically
build the docs as you're working on them [0].

This uses the sphinx-autobuild package [1] but unfortunately seems to
have been broken since its most recent release last year [2].

Specifically, the sphinx-autobuild command line syntax no longer accepts
shorthand options -p and -H, requiring use of --port and --host instead.
Additionally the --poll option is no longer supported.

To test, run `cd docs && make livehtml`.

[0] https://docs.wagtail.io/en/stable/contributing/developing.html#compiling-the-documentation
[1] https://github.com/executablebooks/sphinx-autobuild
[2] https://github.com/executablebooks/sphinx-autobuild/blob/master/NEWS.rst#20200901---2020-09-01

docs/Makefile

@@ -185,6 +185,6 @@ pseudoxml:
 	@echo "Build finished. The pseudo-XML files are in $(BUILDDIR)/pseudoxml."
 
 livehtml:
-	sphinx-autobuild --poll -p 4000 -H 0.0.0.0 -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
+	sphinx-autobuild --port 4000 --host 0.0.0.0 -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html