ob-maxima.el: Add group information to a defcustom.

* ob-maxima.el (org-babel-maxima-command): Add group
information to the defcustom.

lisp/ob-maxima.el

@@ -42,7 +42,8 @@
 
 (defcustom org-babel-maxima-command
   (if (boundp 'maxima-command) maxima-command "maxima")
-  "Command used to call maxima on the shell.")
+  "Command used to call maxima on the shell."
+  :group 'org-babel)
 
 (defun org-babel-maxima-expand (body params)
   "Expand a block of Maxima code according to its header arguments."