Add tracking issue for string_extend_from_within

Author: Sky

library/alloc/src/string.rs

@@ -949,7 +949,7 @@ impl String {
     /// assert_eq!(string, "abcdecdeabecde");
     /// ```
     #[cfg(not(no_global_oom_handling))]
-    #[unstable(feature = "string_extend_from_within", issue = "none")]
+    #[unstable(feature = "string_extend_from_within", issue = "103806")]
     pub fn extend_from_within<R>(&mut self, src: R)
     where
         R: RangeBounds<usize>,