[MLton] Type on error msg.

[Ray Racine]

In and around line 1744 of elaborate-core.fun there is a typo that reads
"bug got" when it was probably meant to be "but got".

You can see a "bug" and "but" example below.

Error: flatten.sml 130.5.
  Recursive use of function disagrees with its type.
    expects: _ -> [flattriple] list
    bug got: _ -> [flatnode] list
    in: statement s = case s of (N3A.Declar  ...  rops propertylist []
end)
Error: main.sml 16.6.
  Function applied to incorrect argument.
    expects: [Flatten.flattriple] list
    but got: [Flatten.flatnode] list
    in: (List.app (fn (t) => (Layout.output  ...  , TextIO.stdOut))))
trips