chore: make format_markdown via npx

Author: tony

Makefile

@@ -51,4 +51,4 @@ watch_flake8:
 	if command -v entr > /dev/null; then ${PY_FILES} | entr -c $(MAKE) flake8; else $(MAKE) flake8 entr_warn; fi
 
 format_markdown:
-	prettier --parser=markdown -w *.md docs/*.md docs/**/*.md CHANGES
+	npx prettier --parser=markdown -w *.md docs/*.md docs/**/*.md CHANGES