Minor simplification in lib.rs doc string

tokio/src/lib.rs

@@ -284,7 +284,7 @@
 //!             loop {
 //!                 let n = match socket.read(&mut buf).await {
 //!                     // socket closed
-//!                     Ok(n) if n == 0 => return,
+//!                     Ok(0) => return,
 //!                     Ok(n) => n,
 //!                     Err(e) => {
 //!                         eprintln!("failed to read from socket; err = {:?}", e);