Refer to add method instead of offset

The `align_offset` method returns an `usize`, so using `add` makes more sense than using `offset`, which takes an `isize`.

src/libcore/ptr/mod.rs

@@ -1534,7 +1534,7 @@ impl<T: ?Sized> *const T {
     /// `usize::max_value()`.
     ///
     /// The offset is expressed in number of `T` elements, and not bytes. The value returned can be
-    /// used with the `offset` method.
+    /// used with the `add` method.
     ///
     /// There are no guarantees whatsover that offsetting the pointer will not overflow or go
     /// beyond the allocation that the pointer points into. It is up to the caller to ensure that
@@ -2335,7 +2335,7 @@ impl<T: ?Sized> *mut T {
     /// `usize::max_value()`.
     ///
     /// The offset is expressed in number of `T` elements, and not bytes. The value returned can be
-    /// used with the `offset` method.
+    /// used with the `add` method.
     ///
     /// There are no guarantees whatsover that offsetting the pointer will not overflow or go
     /// beyond the allocation that the pointer points into. It is up to the caller to ensure that