(desktop-internal-v2s): Make structure match docstring.

lisp/desktop.el

@@ -246,7 +246,7 @@ and evaluated yields value.  quote may be 'may (value may be quoted),
 			" (list 'lambda '() (list 'set-marker mk "
 			pos " (get-buffer " buf ")))) mk)"))))
    (t					; save as text
-    (prin1-to-string (prin1-to-string val)))))
+    (cons nil (prin1-to-string (prin1-to-string val))))))
 
 (defun desktop-value-to-string (val)
   "Convert VALUE to a string that when read evaluates to the same value.  Not