Remove warning from typing.stop()

See https://github.com/serenity-rs/serenity/pull/2081

src/commands/challenge.rs

@@ -169,7 +169,7 @@ async fn submit(ctx: &Context, msg: &Message) -> CommandResult {
     } else if invalid_types {
         message.push_str("Sorry, your submission could not be uploaded; only **.png**, **.jpg**, and **.jpeg** files are permitted.");
     }
-    typing.stop();
+    let _ = typing.stop();
     msg.reply(&ctx.http, message).await?;
     Ok(())
 }