Make `org-show-children' interactive

* lisp/org.el (org-show-children): Make it a command.

Reported-by: Tokuya Kameshima <kametoku@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/101347>

lisp/org.el

@@ -24917,8 +24917,10 @@ 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."
+Prefix arg LEVEL is how many levels below the current level
+should be shown.  Default is enough to cause the following
+heading to appear."
+  (interactive "p")
   (save-excursion
     (org-back-to-heading t)
     (let* ((current-level (funcall outline-level))