Add full stop in NOTE in 1984/mullender/Makefile

This to match other Makefiles with notes which I will also be going
through in time.

1984/mullender/Makefile

@@ -442,7 +442,7 @@ all: ${DATA} ${TARGET}
 	supernova deep_magic magic charon pluto
 
 ${PROG}: ${PROG}.c
-	@echo "NOTE: This entry is likely to fail and/or dump core on modern computers"
+	@echo "NOTE: This entry is likely to fail and/or dump core on modern computers."
 	${CC} ${CFLAGS} $< -o $@ ${LIBS}
 
 # alternative executable