org.texi: Correct suggested value for org-agenda-inhibit-startup

* doc/org.texi (Speeding up your agendas): Adjust
org-agenda-inhibit-startup snippet to match the text.

doc/org.texi

@@ -18995,7 +18995,7 @@ Do not dim blocked tasks:
 @vindex org-agenda-inhibit-startup
 Stop preparing agenda buffers on startup:
 @lisp
-(setq org-agenda-inhibit-startup nil)
+(setq org-agenda-inhibit-startup t)
 @end lisp
 @item
 @vindex org-agenda-show-inherited-tags