Stablize string_extend_from_within

Author: aDotInTheVoid

Diff for: alloc/src/string.rs

@@ -1137,7 +1137,6 @@ impl String {
     /// # Examples
     ///
     /// ```
-    /// #![feature(string_extend_from_within)]
     /// let mut string = String::from("abcde");
     ///
     /// string.extend_from_within(2..);
@@ -1150,7 +1149,7 @@ impl String {
     /// assert_eq!(string, "abcdecdeabecde");
     /// ```
     #[cfg(not(no_global_oom_handling))]
-    #[unstable(feature = "string_extend_from_within", issue = "103806")]
+    #[stable(feature = "string_extend_from_within", since = "CURRENT_RUSTC_VERSION")]
     pub fn extend_from_within<R>(&mut self, src: R)
     where
         R: RangeBounds<usize>,