ob-exp: Fix code indentation after src block evaluation

* lisp/ob-exp.el (org-export-blocks-preprocess): Do not use
  `indent-code-rigidly' as it doesn't indent contents of strings.

lisp/ob-exp.el

@@ -295,7 +295,7 @@ this template."
 					   (goto-char match-start)
 					   (indent-line-to ind))
 			 ;; Indent everything.
-			 (indent-code-rigidly match-start (point) ind)))))
+			 (indent-rigidly match-start (point) ind)))))
 	      (setq pos (line-beginning-position))
               ;; Cleanup markers.
 	      (set-marker match-start nil)