Change "us" to "is" in Layout::for_value comment

Author: kraai

src/libcore/alloc.rs

@@ -137,7 +137,7 @@ impl Layout {
     #[inline]
     pub fn for_value<T: ?Sized>(t: &T) -> Self {
         let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
-        // See rationale in `new` for why this us using an unsafe variant below
+        // See rationale in `new` for why this is using an unsafe variant below
         debug_assert!(Layout::from_size_align(size, align).is_ok());
         unsafe {
             Layout::from_size_align_unchecked(size, align)