org-manual: Correct suggested value for org-agenda-inhibit-startup

* doc/org-manual.org (Speeding Up Your Agendas): Adjust
org-agenda-inhibit-startup snippet to match the text.

This correpsonds to maint's 01c419426.

doc/org-manual.org

@@ -20030,7 +20030,7 @@ number.  Here are tips to speed up:
   #+vindex: org-agenda-inhibit-startup
 
   #+begin_src emacs-lisp
-  (setq org-agenda-inhibit-startup nil)
+  (setq org-agenda-inhibit-startup t)
   #+end_src
 
 - Disable tag inheritance for agendas: