improve grammar

Co-authored-by: Bastian Kauschke <bastian_kauschke@hotmail.de>

src/libcore/mem/mod.rs

@@ -358,7 +358,7 @@ pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
 ///     - an (unstable) [extern type], then this function is always safe to
 ///       call, but may panic or otherwise return the wrong value, as the
 ///       extern type's layout is not known. This is the same behavior as
-///       [`size_of_val`] on a reference to a type with extern type tail.
+///       [`size_of_val`] on a reference to a type with an extern type tail.
 ///     - otherwise, it is conservatively not allowed to call this function.
 ///
 /// [slice]: ../../std/primitive.slice.html
@@ -493,7 +493,7 @@ pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
 ///     - an (unstable) [extern type], then this function is always safe to
 ///       call, but may panic or otherwise return the wrong value, as the
 ///       extern type's layout is not known. This is the same behavior as
-///       [`align_of_val`] on a reference to a type with extern type tail.
+///       [`align_of_val`] on a reference to a type with an extern type tail.
 ///     - otherwise, it is conservatively not allowed to call this function.
 ///
 /// [slice]: ../../std/primitive.slice.html