Change example code to not use an actual control character.

doc/faq.rst

@@ -215,7 +215,7 @@ Info files, try adding the following Emacs Lisp code to your start-up file,
          (widen) (goto-char (point-min))
          (when (re-search-forward
                 "^Generated by \\(Sphinx\\|Docutils\\)"
-                (save-excursion (search-forward "^_" nil t)) t)
+                (save-excursion (search-forward "\x1f" nil t)) t)
            (set (make-local-variable 'Info-hide-note-references)
                 'hide)))))