Fix org-agenda-prefix-format docstring

* org-agenda.el (org-agenda-prefix-format): Add missing `search' item
  in docstring.

Signed-off-by: Julien Danjou <julien@danjou.info>

lisp/org-agenda.el

@@ -1303,7 +1303,7 @@ When nil, such items are sorted as 0 minutes effort."
     (search . " %i %-12:c"))
   "Format specifications for the prefix of items in the agenda views.
 An alist with four entries, for the different agenda types.  The keys to the
-sublists are `agenda', `timeline', `todo', and `tags'.  The values
+sublists are `agenda', `timeline', `todo', `search' and `tags'.  The values
 are format strings.
 This format works similar to a printf format, with the following meaning: