[ptr.align] uses uppercase X for an object

[jwakely]

The library conventionally uses lowercase letters for objects. This was pointed out by Alisdair during the review (San Diego 2018) but the author said the choice of X was based on the wording for launder. However, [ptr.launder] uses a math font X which is a little more obvious it's not using our usual convention for types.

Can we either make it x or X, but not X?