Fix invalid date for theorem canth.
Fix invalid date for theorem canth.

The "canth" theory said it was contributed by NM on 1-Apr-198,
but that year is improbable.  The Google docs document tracking the
metamath 100 reports its contribution date as
1994-08-07 (7-Aug-1994), so use that date instead.

metamath/set.mm,metamath/set.mm,metamath/set.mm