Use if-let in std::result documentation

This takes an opportunity to show yet-another-way of dealing with
Results and matching.

Author: nagisa

src/libcore/result.rs

@@ -178,13 +178,11 @@
 //! fn write_info(info: &Info) -> Result<(), IoError> {
 //!     let mut file = File::open_mode(&Path::new("my_best_friends.txt"), Open, Write);
 //!     // Early return on error
-//!     match file.write_line(format!("name: {}", info.name).as_slice()) {
-//!         Ok(_) => (),
-//!         Err(e) => return Err(e)
+//!     if let Err(e) = file.write_line(format!("name: {}", info.name).as_slice()) {
+//!         return Err(e)
 //!     }
-//!     match file.write_line(format!("age: {}", info.age).as_slice()) {
-//!         Ok(_) => (),
-//!         Err(e) => return Err(e)
+//!     if let Err(e) = file.write_line(format!("age: {}", info.age).as_slice()) {
+//!         return Err(e)
 //!     }
 //!     return file.write_line(format!("rating: {}", info.rating).as_slice());
 //! }