[commands] Fix documented type of Context.cog

discord/ext/commands/context.py

@@ -204,7 +204,7 @@ class Context(discord.abc.Messageable):
 
     @property
     def cog(self):
-        """:class:`.Cog`: Returns the cog associated with this context's command. None if it does not exist."""
+        """Optional[:class:`.Cog`]: Returns the cog associated with this context's command. None if it does not exist."""
 
         if self.command is None:
             return None