org-bbdb: fix anniversary with format MM-DD

* lisp/org-bbdb.el (org-bbdb-anniv-extract-date): Fix the pattern to
  be able to match a pair.

Reported by

From: leandro@disroot.org
Subject: Anniversaries from BBDB
Newsgroups: gmane.emacs.orgmode
To: emacs-orgmode@gnu.org
Date: Thu, 20 Sep 2018 22:08:12

lisp/org-bbdb.el

@@ -300,7 +300,7 @@ italicized, in all other cases it is left unchanged."
 Argument TIME-STR is the value retrieved from BBDB.  If YYYY- is omitted
 it will be considered unknown."
   (pcase (org-split-string time-str "-")
-    (`(,a ,b nil) (list (string-to-number a) (string-to-number b) nil))
+    (`(,a ,b) (list (string-to-number a) (string-to-number b) nil))
     (`(,a ,b ,c) (list (string-to-number b)
 		       (string-to-number c)
 		       (string-to-number a)))))