org-agenda: Fix missing colons in agenda tags

* lisp/org-agenda.el (org-agenda-format-item): Fix missing colons in
  agenda tags.

This regression was introduced in "Standardize tag regexps" commit.

Reported-by: Samuel Wales <samologist@gmail.com>
<http://lists.gnu.org/r/emacs-orgmode/2018-04/msg00321.html>

lisp/org-agenda.el

@@ -6530,7 +6530,7 @@ Any match of REMOVE-RE will be removed from TXT."
 	      (setq txt (replace-match "" t t txt))
 	    (setq txt (replace-match
 		       (concat (make-string (max (- 50 (length txt)) 1) ?\ )
-			       (match-string 2 txt))
+			       (match-string 1 txt))
 		       t t txt))))
 
 	(when remove-re