fix double wording

Author: DutchGhost

src/libcore/slice/mod.rs

@@ -645,7 +645,7 @@ impl<T> [T] {
     ///
     /// See [`chunks_exact_mut`] for a variant of this iterator that returns chunks of always
     /// exactly `chunk_size` elements, and [`rchunks_mut`] for the same iterator but starting at
-    /// the end of the slice of the slice.
+    /// the end of the slice.
     ///
     /// # Panics
     ///