lisp/org-agenda.el: Use "09:00" instead of " 9:00"

* lisp/org-agenda.el (org-get-time-of-day): Use "09:00" instead
of " 9:00" when displaying as a string.

This makes the string more readable, especially for users reading
the agenda with a screen reader.

Reported-by: Sébastien Hinderer

lisp/org-agenda.el

@@ -6995,7 +6995,7 @@ HH:MM."
 	     (h2 (if (and string mod24 (not (and (= m 0) (= h1 24))))
 		     (mod h1 24) h1))
 	     (t0 (+ (* 100 h2) m))
-	     (t1 (concat (if (>= h1 24) "+" " ")
+	     (t1 (concat (if (>= h1 24) "+" "0")
 			 (if (and org-agenda-time-leading-zero
 				  (< t0 1000)) "0" "")
 			 (if (< t0 100) "0" "")