org-src: Tiny fix

* lisp/org-src.el (org-src-font-lock-fontify-block): Ensure LANG is case
  insensitive.

lisp/org-src.el

@@ -513,7 +513,7 @@ as `org-src-fontify-natively' is non-nil."
 		   org-buffer)))
 	      (setq pos next))))
 	;; Add Org faces.
-	(let ((src-face (cadr (assoc-string lang org-src-block-faces))))
+	(let ((src-face (nth 1 (assoc-string lang org-src-block-faces t))))
           (when (or (facep src-face) (listp src-face))
             (font-lock-append-text-property start end 'face src-face))
 	  (font-lock-append-text-property start end 'face 'org-block))