Fix comment in mto
Fix comment in mto

The comment in theorem ~ mto was wrong.  Fix it, and while we're at it,
note that this rule is also valid in intuitionistic logic.

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

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