Add a missing article

Add a missing article

library/core/src/convert/mod.rs

@@ -112,7 +112,7 @@ pub const fn identity<T>(x: T) -> T {
 ///
 /// - Unlike `AsRef`, [`Borrow`] has a blanket impl for any `T`, and can be used to accept either
 ///   a reference or a value.
-/// - [`Borrow`] also requires that [`Hash`], [`Eq`] and [`Ord`] for borrowed value are
+/// - [`Borrow`] also requires that [`Hash`], [`Eq`] and [`Ord`] for a borrowed value are
 ///   equivalent to those of the owned value. For this reason, if you want to
 ///   borrow only a single field of a struct you can implement `AsRef`, but not [`Borrow`].
 ///