Update slice_rotate to a real tracking number

Author: scottmcm

src/doc/unstable-book/src/library-features/slice-rotate.md

@@ -1,7 +1,7 @@
 # `slice_rotate`
 
-The tracking issue for this feature is: [#123456789]
+The tracking issue for this feature is: [#41891]
 
-[#123456789]: https://github.com/rust-lang/rust/issues/123456789
+[#41891]: https://github.com/rust-lang/rust/issues/41891
 
 ------------------------

src/libcollections/slice.rs

@@ -1387,7 +1387,7 @@ impl<T> [T] {
     /// slide(&mut v, 6..8, 1);
     /// assert_eq!(&v, &[0, 3, 7, 4, 5, 6, 1, 2, 8, 9]);
     /// ```
-    #[unstable(feature = "slice_rotate", issue = "123456789")]
+    #[unstable(feature = "slice_rotate", issue = "41891")]
     pub fn rotate(&mut self, mid: usize) -> usize {
         core_slice::SliceExt::rotate(self, mid)
     }

src/libcore/slice/mod.rs

@@ -203,7 +203,7 @@ pub trait SliceExt {
     #[stable(feature = "core", since = "1.6.0")]
     fn ends_with(&self, needle: &[Self::Item]) -> bool where Self::Item: PartialEq;
 
-    #[unstable(feature = "slice_rotate", issue = "123456789")]
+    #[unstable(feature = "slice_rotate", issue = "41891")]
     fn rotate(&mut self, mid: usize) -> usize;
 
     #[stable(feature = "clone_from_slice", since = "1.7.0")]