fix bug

the `msg` type here is `Message`, don't need `unwrap()`, then wrapping `msg.content` in `Some`, return to fit the `filter_map`'s trait bound: Option<T>

src/async-rust/tokio/stream.md

@@ -184,7 +184,7 @@ got = b"6"
 let messages = subscriber
     .into_stream()
     .filter_map(|msg| match msg {
-        Ok(msg) if msg.content.len() == 1 => msg.unwrap().content,
+        Ok(msg) if msg.content.len() == 1 => Some(msg.content),
         _ => None,
     })
     .take(3);