[Microsoft.DeviceUpdate] Fixed suppressions in readme.md (#20019)

Author: darkoa-msft

specification/deviceupdate/resource-manager/readme.md

@@ -67,7 +67,7 @@ directive:
     reason: Managed Identity type can be case in-sensitive
   - suppress: BodyTopLevelProperties
     from: deviceupdate.json
-    where: $.definitions.PrivateEndpointConnectionProxy
+    where: $.definitions.PrivateEndpointConnectionProxy.properties
     reason: Internal NRP resource, all properties are top level properties
 ```