std::io: Use re-exported pathes in examples.

We use re-exported pathes (e.g. std::io::Command) and original ones
(e.g. std::io::process::Command) together in examples now. Using
re-exported ones consistently avoids confusion.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>

src/libstd/io/mod.rs

@@ -83,7 +83,7 @@ Some examples of obvious things you might want to do
 
     ```rust
     # #![allow(unused_must_use)]
-    use std::io::net::tcp::TcpStream;
+    use std::io::TcpStream;
 
     # // connection doesn't fail if a server is running on 8080
     # // locally, we still want to be type checking this code, so lets