enable multiprocessing for pyspelling

Pyspelling has a new option to run jobs in parallel. See
facelessuser/pyspelling#182

This doesn't make much difference (if at all) for the starter
pack, but it's considerably faster for large documentation sets.

Signed-off-by: Ruth Fuchss <ruth.fuchss@canonical.com>

Makefile

@@ -83,7 +83,7 @@ clean-doc:
 	rm -rf .sphinx/.doctrees
 
 spelling: html
-	. $(VENV) ; python3 -m pyspelling -c .sphinx/spellingcheck.yaml
+	. $(VENV) ; python3 -m pyspelling -c .sphinx/spellingcheck.yaml -j $(shell nproc)
 
 linkcheck: install
 	. $(VENV) ; $(SPHINXBUILD) -b linkcheck "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS)