Fixup .mailmap file

Fix a few issues with the `.mailmap` file.

(cherry picked from commit 3a5de00c)

.mailmap

@@ -6,7 +6,8 @@ Juergen Hoeller <jhoeller@vmware.com> jhoeller <jhoeller@vmware.com>
 <apoutsma@gopivotal.com> <apoutsma@vmware.com>
 <apoutsma@gopivotal.com> <poutsma@mac.com>
 <ogierke@gopivotal.com> <ogierke@vmware.com>
-<aclement@gopivotal.com> <david_syer@hotmail.com>
+<dsyer@gopivotal.com> <david_syer@hotmail.com>
+<dsyer@gopivotal.com> <dsyer@vmware.com>
 <aclement@gopivotal.com> <andrew.clement@gmail.com>
 <aclement@gopivotal.com> <aclement@vmware.com>
 <dmitry.katsubo@gmail.com> <dmitry.katsubo@gmai.com>