Fix all "31-Jun" dates (should be 30-Jun)
Fix all "31-Jun" dates (should be 30-Jun)

There are only 30 days in June. Fix all "31-Jun" dates to
say "30-Jun" instead in set.mm.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

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