Add leading _ so that motd file isn't loaded as a Cog

Author: ChrisLovering

arthur/exts/systems/motd.py renamed to arthur/exts/systems/_motd.py

@@ -19,7 +19,7 @@
 from arthur.apis.systems import lib9front
 from arthur.bot import KingArthur
 from arthur.config import CONFIG
-from arthur.exts.systems.motd import MOTD
+from arthur.exts.systems._motd import MOTD
 
 BASE_RESOURCE = "https://git.9front.org/plan9front/plan9front/HEAD/{}/raw"
 THRESHOLD = 0.01