Backport: Fix name of gnus-summary-sort-by-mark(s)

* lisp/gnus/gnus-sum.el (gnus-summary-sort-by-marks): Needs the
  "s", according to docs and keymap both. (bug#35765)

(cherry picked from commit 13248f74)

lisp/gnus/gnus-sum.el

@@ -11963,7 +11963,7 @@ Argument REVERSE means reverse order."
   (interactive "P")
   (gnus-summary-sort 'chars reverse))
 
-(defun gnus-summary-sort-by-mark (&optional reverse)
+(defun gnus-summary-sort-by-marks (&optional reverse)
   "Sort the summary buffer by article marks.
 Argument REVERSE means reverse order."
   (interactive "P")