Fixed async method

CheckPermissions was not set to async so await would not work.

docs/guides/samples/require_owner.cs

@@ -4,7 +4,7 @@
 public class RequireOwnerAttribute : PreconditionAttribute
 {
     // Override the CheckPermissions method
-    public override Task<PreconditionResult> CheckPermissions(CommandContext context, CommandInfo command, IDependencyMap map)
+    public async override Task<PreconditionResult> CheckPermissions(CommandContext context, CommandInfo command, IDependencyMap map)
     {
         // Get the ID of the bot's owner
         var ownerId = (await map.Get<DiscordSocketClient>().GetApplicationInfoAsync()).Owner.Id;
@@ -15,4 +15,4 @@ public class RequireOwnerAttribute : PreconditionAttribute
         else
             return PreconditionResult.FromError("You must be the owner of the bot to run this command.");
     }
-}
+}