update wording

Author: pietroalbini

RELEASES.md

@@ -91,7 +91,7 @@ Stabilized APIs
 - [`<str>::from_utf8_unchecked`](https://doc.rust-lang.org/stable/std/primitive.str.html#method.from_utf8_unchecked)
 - [`<str>::from_utf8_unchecked_mut`](https://doc.rust-lang.org/stable/std/primitive.str.html#method.from_utf8_unchecked_mut)
 
-These APIs are now stable in const contexts:
+These previously stable APIs are now stable in const contexts:
 
 - [`core::str::from_utf8_mut`](https://doc.rust-lang.org/stable/std/str/fn.from_utf8_mut.html)
 - [`<[T]>::copy_from_slice`](https://doc.rust-lang.org/stable/std/primitive.slice.html#method.copy_from_slice)