Reference implementation update

Ken Dickey has fixed the following problems with the reference implementation
for this SRFI.  This changes are reflected in an updated version of the document.

The changes are:
    [1] "~h" now prints the appropriate header line in the documentation string.
    [2] "~w,dF" now rounds appropriately
        e.g: (format "~8,2F" 3.4567e11) --> " 3.46e11", not "4.45e11"
    [3] Format now reports an error if there are too few arguments
        e.g. for (format "~a")

David