Fix wrong documented permission for GuildChannel.invites()

I tested it just now, and manage_guild is not the permission you need to
fetch invites from a given channel. You need manage_channels.

Note that this isn't the same as Guild.invites(), which DOES use
manage_guild.

discord/abc.py

@@ -755,7 +755,7 @@ class GuildChannel:
 
         Returns a list of all active instant invites from this channel.
 
-        You must have :attr:`~Permissions.manage_guild` to get this information.
+        You must have :attr:`~Permissions.manage_channels` to get this information.
 
         Raises
         -------