Fix doxigen comment after changed const behaviour

src/libqof/qof/qofbook.h

@@ -333,7 +333,7 @@ gchar * qof_book_normalize_counter_format(const gchar *format, gchar **err_msg);
 
 /** Get the format string to use for the named counter.
  *    The return value is NULL on error or the format string of the
- *    counter. The string should not be freed.
+ *    counter. The returned string should be freed by the caller.
  */
 gchar *qof_book_get_counter_format (const QofBook *book, const char *counter_name);