Rename slice_from_ptr_range_const -> const_slice_from_ptr_range

This is in line with other `const fn` features.

Author: WaffleLapkin

core/src/slice/raw.rs

@@ -213,7 +213,7 @@ pub const fn from_mut<T>(s: &mut T) -> &mut [T] {
 ///
 /// [valid]: ptr#safety
 #[unstable(feature = "slice_from_ptr_range", issue = "89792")]
-#[rustc_const_unstable(feature = "slice_from_ptr_range_const", issue = "89792")]
+#[rustc_const_unstable(feature = "const_slice_from_ptr_range", issue = "89792")]
 pub const unsafe fn from_ptr_range<'a, T>(range: Range<*const T>) -> &'a [T] {
     // SAFETY: the caller must uphold the safety contract for `from_ptr_range`.
     unsafe { from_raw_parts(range.start, range.end.sub_ptr(range.start)) }