Update mgidx.yaml

Porting mgidx changes per #3050

Note: I believe adapters are disabled by default in PBS-Java, so didn't port that element.

src/main/resources/bidder-config/mgidx.yaml

@@ -1,6 +1,8 @@
 adapters:
   mgidX:
-    endpoint: https://us-east-x.mgid.com/pserver
+    # We have the following regional endpoint domains: 'us-east-x' and 'eu'
+    # Please deploy this config in each of your datacenters with the appropriate regional subdomain
+    endpoint: "https://REGION.mgid.com/pserver"
     meta-info:
       maintainer-email: prebid@mgid.com
       app-media-types: