fix doc examples

tokio/src/task/builder.rs

@@ -48,7 +48,7 @@ use std::{future::Future, io};
 ///             .spawn(async move {
 ///                 // Process each socket concurrently.
 ///                 process(socket).await
-///             });
+///             })?;
 ///     }
 /// }
 /// ```

tokio/src/task/join_set.rs

@@ -101,13 +101,13 @@ impl<T: 'static> JoinSet<T> {
     /// use tokio::task::JoinSet;
     ///
     /// #[tokio::main]
-    /// async fn main() {
+    /// async fn main() -> io::Result<()> {
     ///     let mut set = JoinSet::new();
     ///
     ///     // Use the builder to configure a task's name before spawning it.
     ///     set.build_task()
     ///         .name("my_task")
-    ///         .spawn(async { /* ... */ });
+    ///         .spawn(async { /* ... */ })?;
     /// }
     /// ```
     #[cfg(all(tokio_unstable, feature = "tracing"))]