don't use -M with sphinx-build

build-docs.sh

@@ -5,4 +5,4 @@ python docs/dump-openapi-spec.py
 #   ... and install dot on the system
 # python docs/render_db_diagrams.py
 
-sphinx-build -M html docs/source docs/build
+sphinx-build -b html docs/source docs/build

tox.ini

@@ -20,4 +20,4 @@ commands =
     #   pip install -r docs/requirements-er.txt
     #   ... and install dot on the system
     # python docs/render_db_diagrams.py
-    sphinx-build -M html docs/source docs/build
+    sphinx-build -b html docs/source docs/build