Makefile: prettier -> format_markdown

Author: tony

Makefile

@@ -40,5 +40,5 @@ flake8:
 watch_flake8:
 	if command -v entr > /dev/null; then ${PY_FILES} | entr -c $(MAKE) flake8; else $(MAKE) flake8 entr_warn; fi
 
-prettier:
+format_markdown:
 	prettier --parser=markdown -w *.md docs/*.md docs/**/*.md CHANGES