[MLton] Bug with "polymorphic" exns

[Matthew Fluet]

> PS: Btw., the mail link on http://mlton.org/Bug contains a trailing dot.

Thanks, fixed.

BTW, to other Wiki editors, I've regularized the pages on
 [mailto MLton@mlton.org MLton@mlton.org]
in "send mail to" contexts and
 {{{MLton@mlton.org}}}
in other noun contexts (like Contacts).