babel: R: Fix bug in treatment of :file argument

	* ob-R.el (org-babel-expand-body:R): Fix bug in let binding.

This was preventing graphical output from being captured.

lisp/ob-R.el

@@ -53,7 +53,7 @@
 
 (defun org-babel-expand-body:R (body params)
   "Expand BODY according to PARAMS, return the expanded body."
-  (let (out-file (cdr (assoc :file params)))
+  (let ((out-file (cdr (assoc :file params))))
     (mapconcat
      #'identity
      ((lambda (inside)