OWNERS: Update Gunnar's email address

I have multiple email addresses in my Gerrit account but as
https://gerrit.openbmc.org/c/openbmc/bmcweb/+/69506 shows my Preferred
email has to match the OWNERS file, do that.

More discussion in Discord at
[1] https://discord.com/channels/775381525260664832/817151035197358081/1209585278025601094

Tested: With https://gerrit.openbmc.org/c/openbmc/bmcweb/+/69549 my +2
means something again.

Change-Id: I091a5cc00acff4c268a86c6c9c79f705992738af
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/designs/oem/ibm/OWNERS b/designs/oem/ibm/OWNERS
index e7b96a6..76ec9cf 100644
--- a/designs/oem/ibm/OWNERS
+++ b/designs/oem/ibm/OWNERS
@@ -33,7 +33,7 @@
 
 owners:
 - geissonator@yahoo.com
-- gmills@linux.vnet.ibm.com
+- gunnar@gmills.xyz
 
 reviewers:
 
@@ -44,5 +44,5 @@
   email: geissonator@yahoo.com
   discord: geissonator
 - name: Gunnar Mills
-  email: gmills@linux.vnet.ibm.com
+  email: gunnar@gmills.xyz
   discord: GunnarM