Fix bug from different definition of ObjectID (#20095)

types/meteor/mongo.d.ts

@@ -103,7 +103,10 @@ declare module Mongo {
     interface ObjectIDStatic {
         new (hexString?: string): ObjectID;
     }
-    interface ObjectID { }
+    interface ObjectID {
+        toHexString(): string;
+        equals(otherID: ObjectID): boolean;
+    }
 
     function setConnectionOptions(options: any): void;
 }