Rollup merge of #155493 - sorairolake:fix-alignment-doc-link, r=scottmcm

docs(num): fix stale link to `mem::Alignment`

This pull request updates a stale link to `mem::Alignment` in `num::IntErrorKind`.

In rust-lang/rust#153178, I added a link to `Alignment` in `IntErrorKind`, but I overlooked that `Alignment` had been moved from `core::ptr` to `core::mem`. Although it is still re-exported in `core::ptr`, this pull request points the link to its canonical location.

@rustbot label +A-docs

Author: JonathanBrouwer