Fix error in example by adding type annotation

src/liballoc/boxed.rs

@@ -479,7 +479,7 @@ impl<'a, T: Copy> From<&'a [T]> for Box<[T]> {
     /// ```rust
     /// // create a &[u8] which will be used to create a Box<[u8]>
     /// let slice: &[u8] = &[104, 101, 108, 108, 111];
-    /// let boxed_slice = Box::from(slice);
+    /// let boxed_slice: Box<[u8]> = Box::from(slice);
     ///
     /// println!("{:?}", boxed_slice);
     /// ```