Fix the date prompt for american-style dates

Fix the date prompt for american-style dates

* lisp/org.el (org-read-date-analyze): Fix regular expression for
matching american dates

Daniel E. Doherty writes:

> In playing around with the date prompt (C-.), I ran across the following
> puzzling behavior from rather simple inputs.
>
> I entered the following on June 1, 2010.  Here is a date entered as
> "3/15": <2011-03-15 Tue>.  It interpreted it as the upcoming March 15 as
> expected.
>
> But here is a date entered as "5/21": <2021-06-05 Sat>.  Note how it
> interpreted the "21" as the year 2021, not at all what I expected from
> the documentation or the analogous "3/15" example.
>
> Maybe there is some underlying logic here that I'm not getting.  Perhaps
> it has to do with how 2-digit years are interpreted?
>
> What's going on here?  I am using org-version 6.36trans on emacs 23.1.

What was going on here is that the regular expression for matching
american-style dates was wrong.  It was looking for month numbers in
the second field and day numbers in the first field - wrong, of
course.