Fix a docstring

* lisp/org.el (org-show-children): Fix docstring.

lisp/org.el

@@ -24917,8 +24917,9 @@ modified."
 
 (defun org-show-children (&optional level)
   "Show all direct subheadings of this heading.
-Prefix arg LEVEL is how many levels below the current level should be shown.
-Default is enough to cause the following heading to appear."
+Optional argument LEVEL is how many levels below the current
+level should be shown.  Default is enough to cause the following
+heading to appear."
   (save-excursion
     (org-back-to-heading t)
     (let* ((current-level (funcall outline-level))