Also fix the signature of main in std::sys::unix::ext

src/libstd/sys/unix/ext/mod.rs

@@ -15,11 +15,13 @@
 //! use std::fs::File;
 //! use std::os::unix::prelude::*;
 //!
-//! fn main() {
+//! fn main() -> std::io::Result<()> {
 //!     let f = File::create("foo.txt")?;
 //!     let fd = f.as_raw_fd();
 //!
 //!     // use fd with native unix bindings
+//!
+//!     Ok(())
 //! }
 //! ```