org-agenda.el: Complete multiple todo keywords

* lisp/org-agenda.el (org-todo-list): Use completing-read-multiple
  instead of completing-read when selecting todo keywords to filter by
  in Agenda.  Fix a typo in the prompt.


--- a/lisp/org-agenda.el
+++ b/lisp/org-agenda.el
@@ -4790,8 +4790,12 @@
                                                 (nth (1- arg) kwds))))
       (when (equal arg '(4))
         (setq org-select-this-todo-keyword
-              (completing-read "Keyword (or KWD1|K2D2|...): "
-                               (mapcar #'list kwds) nil nil)))
+              (mapconcat #'identity
+                         (let ((crm-separator "|"))
+                           (completing-read-multiple
+                            "Keyword (or KWD1|KWD2|...): "
+                            (mapcar #'list kwds) nil nil))
+                         "|")))
       (and (equal 0 arg) (setq org-select-this-todo-keyword nil))
       (org-compile-prefix-format 'todo)
       (org-set-sorting-strategy 'todo)