Fix typo in Makefile: PYTEST_ADDOPT -> PYTEST_ADDOPTS

Author: nicoddemus

doc/en/Makefile

@@ -39,7 +39,7 @@ clean:
 	-rm -rf $(BUILDDIR)/*
 
 regen:
-	PYTHONDONTWRITEBYTECODE=1 PYTEST_ADDOPT=-pno:hypothesis COLUMNS=76 regendoc --update *.rst */*.rst ${REGENDOC_ARGS}
+	PYTHONDONTWRITEBYTECODE=1 PYTEST_ADDOPTS=-pno:hypothesis COLUMNS=76 regendoc --update *.rst */*.rst ${REGENDOC_ARGS}
 
 html:
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html