io::Seek::rewind: Set tracking issue

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

Author: ijackson

library/std/src/io/mod.rs

@@ -1698,7 +1698,7 @@ pub trait Seek {
     /// f.read_to_string(&mut buf).unwrap();
     /// assert_eq!(&buf, hello);
     /// ```
-    #[unstable(feature = "seek_rewind", issue = "none")]
+    #[unstable(feature = "seek_rewind", issue = "85149")]
     fn rewind(&mut self) -> Result<()> {
         self.seek(SeekFrom::Start(0))?;
         Ok(())