Fix order of arguments to HASH-TABLE/MODIFY!.

Fix order of arguments to HASH-TABLE/MODIFY!.

With any luck, nobody was using this except me outside the tree.
And it's not documented, so that nobody character will get what he
deserves.

Before:

   (hash-table/modify! hash-table key
     (lambda ()
       ...
         a gigantic
         heffable hoary herralump of
         an expression
       ...)
     tiny)

After:

   (hash-table/modify! hash-table key tiny
     (lambda ()
       ...
         a gigantic
         heffable hoary herralump of
         an expression
       ...))