net: fix typo in TcpStream internal comment (#6944)

Author: wyfo

tokio/src/net/tcp/stream.rs

@@ -1278,7 +1278,7 @@ impl TcpStream {
 
     // == Poll IO functions that takes `&self` ==
     //
-    // To read or write without mutable access to the `UnixStream`, combine the
+    // To read or write without mutable access to the `TcpStream`, combine the
     // `poll_read_ready` or `poll_write_ready` methods with the `try_read` or
     // `try_write` methods.