* trailing whitespace

* langref: undefined _is_ materialized in all safe modes

I am also not super happy about the clause that immediately follows. I
_believe_ what we want to say here is that, simultaneously:

* undefined is guaranteed to be matrerialized in in all safe modes.
  A Zig implementation that elides `ptr.* = undefined` in ReleaseSafe
  mode would be a non-conforming implementation.
* A Zig program that relies on undefined being materialized is buggy.

But I don't think it's the time to engage this level of language-lawering!