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

Change-Id: I94ba5bd8d9c2a3c23314aae051b3a083562e43cb
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/OWNERS b/OWNERS
index 858cdb5..4c95078 100644
--- a/OWNERS
+++ b/OWNERS
@@ -63,7 +63,7 @@
 
 owners:
 - ed@tanous.net
-- gmills@linux.vnet.ibm.com
+- gunnar@gmills.xyz
 
 
 # The below specifies a list of reviewers and interested parties that should be