Remove the closing bracket for the doc test no-wasm block

Co-authored-by: Alice Ryhl <[email protected]>

Author: martin-g

tokio/src/task/join_set.rs

@@ -656,8 +656,7 @@ where
 /// # Examples
 ///
 /// ```
-/// # #[cfg(not(target_family = "wasm"))]
-/// # {
+/// # #![cfg(not(target_family = "wasm"))]
 /// use tokio::task::JoinSet;
 ///
 /// #[tokio::main]