Hi,

I find that the doc string of `org-at-timestamp-p' about match groups
is not consistent with the behavior. The doc string says

When matching, the match groups are the following:
  group 1: year, if any
  group 2: month, if any
  group 3: day number, if any
  group 4: day name, if any
  group 5: hours, if any
  group 6: minutes, if any

but for timestamps like <2023-08-23 Wed>, (when (org-at-timestamp-p)
(match-string 2)) will return the year 2023 instead of the month. This
patch corrects the group numbers.