Replace deprecated ref field with refs field

R=phosek@google.com, tandrii@google.com

No-Try: true
No-Tree-Checks: true
Bug: 836232
Change-Id: Ifa78fafc8fd71c26204d09ac5bfd137e1d368f71
Reviewed-on: https://gn-review.googlesource.com/2060
Reviewed-by: Petr Hosek <phosek@google.com>
Commit-Queue: Petr Hosek <phosek@google.com>

luci-milo.cfg

diff --git a/luci-milo.cfg b/luci-milo.cfg
index a076e37..3323812 100644
--- a/luci-milo.cfg
+++ b/luci-milo.cfg
@@ -4,7 +4,7 @@
   id: "gn"
   name: "gn"
   repo_url: "https://gn.googlesource.com/gn"
-  ref: "refs/heads/master"
+  refs: "refs/heads/master"
   manifest_name: "REVISION"
   favicon_url: "https://storage.googleapis.com/chrome-infra-public/logo/favicon.ico"