org-exp-blocks.el: Fix regexp for matching the end of a block.

* org-exp-blocks.el (org-export-blocks-preprocess): Fix regexp
for matching the end of a block.

Thanks to Robert Lupton the Good for reporting this and to Nick
Dokos for digging the issue further.

lisp/org-exp-blocks.el

@@ -178,7 +178,7 @@ which defaults to the value of `org-export-blocks-witheld'."
 	    (let* ((match-start (copy-marker (match-beginning 0)))
 		   (body-start (copy-marker (match-end 0)))
 		   (indentation (length (match-string 1)))
-		   (inner-re (format "[\r\n][ \t]*#\\+\\(begin\\|end\\)_%s"
+		   (inner-re (format "^[ \t]*#\\+\\(begin\\|end\\)_%s"
 				     (regexp-quote (downcase (match-string 2)))))
 		   (type (intern (downcase (match-string 2))))
 		   (headers (save-match-data