Add tracking issue number for fmt_as_str.

Author: m-ou-se

src/libcore/fmt/mod.rs

@@ -439,7 +439,7 @@ impl<'a> Arguments<'a> {
     /// assert_eq!(format_args!("").as_str(), Some(""));
     /// assert_eq!(format_args!("{}", 1).as_str(), None);
     /// ```
-    #[unstable(feature = "fmt_as_str", issue = "none")]
+    #[unstable(feature = "fmt_as_str", issue = "74442")]
     #[inline]
     pub fn as_str(&self) -> Option<&'static str> {
         match (self.pieces, self.args) {