org-src.el: Add plain to org-src-window-setup customization

* lisp/org-src.el (org-src-window-setup): Add plain to the list of
valid choices for `org-src-window-setup'.

A previous change added it to the documentation and to the pcase that
process this variable in `org-src-switch-to-buffer', but the defcustom
did not include it as a valid option.

TINYCHANGE

lisp/org-src.el

@@ -171,6 +171,7 @@ Values that modify the window layout (reorganize-frame, split-window-below,
 split-window-right) will restore the layout after exiting the edit buffer."
   :group 'org-edit-structure
   :type '(choice
+          (const plain)
 	  (const current-window)
 	  (const split-window-below)
 	  (const split-window-right)