Fix example for BufferedReader

Fixes #18197

src/libstd/io/buffered.rs

@@ -130,14 +130,13 @@ impl<R: Reader> Reader for BufferedReader<R> {
 /// # Example
 ///
 /// ```rust
-/// # #![allow(unused_must_use)]
 /// use std::io::{BufferedWriter, File};
 ///
-/// let file = File::open(&Path::new("message.txt"));
+/// let file = File::create(&Path::new("message.txt")).unwrap();
 /// let mut writer = BufferedWriter::new(file);
 ///
-/// writer.write_str("hello, world");
-/// writer.flush();
+/// writer.write_str("hello, world").unwrap();
+/// writer.flush().unwrap();
 /// ```
 pub struct BufferedWriter<W> {
     inner: Option<W>,