Fix usage

Fixes #2

unit.py

@@ -14,7 +14,7 @@ class Unit(BotPlugin):
     def convert(self, message, args):
         """Convert units"""
         if not args or ' to ' not in args:
-            return "**Usage**: !unit <from> to <to>"
+            return "**Usage**: !convert <from> to <to>"
 
         src, dst = args.split(' to ')
         try: