Make examples equivalent

The example with the ? operator was missing file.write_all

src/libcore/macros.rs

@@ -330,6 +330,7 @@ macro_rules! debug_assert_ne {
 /// // The prefered method of quick returning Errors
 /// fn write_to_file_question() -> Result<(), MyError> {
 ///     let mut file = File::create("my_best_friends.txt")?;
+///     file.write_all(b"This is a list of my best friends.")?;
 ///     Ok(())
 /// }
 ///