io: rename buffer to file in doc-test (#4230)

tokio/src/io/util/async_write_ext.rs

@@ -353,9 +353,9 @@ cfg_io_util! {
         ///
         /// #[tokio::main]
         /// async fn main() -> io::Result<()> {
-        ///     let mut buffer = File::create("foo.txt").await?;
+        ///     let mut file = File::create("foo.txt").await?;
         ///
-        ///     buffer.write_all(b"some bytes").await?;
+        ///     file.write_all(b"some bytes").await?;
         ///     Ok(())
         /// }
         /// ```