message: docs(common-plugins): fix example
diff: @@ -20,7 +20,11 @@ class SomeClass {
  public populateField: Ref<SomeReferencedClass>;
  }
  -class SomeReferencedClass {}
  +class SomeReferencedClass {
  + // a dummy property is required, otherwise the class will equal to others
  + @prop()
  + public dummy?: string;
  +}
  const SomeClassModel = getModelForClass(SomeClass);
  const SomeReferencedClassModel = getModelForClass(SomeReferencedClass);
  @@ -28,7 +32,7 @@ const SomeReferencedClassModel = getModelForClass(SomeReferencedClass);
  (async () => {
  await mongoose.connect(`mongodb://localhost:27017/`, { useNewUrlParser: true, dbName: 'guides', useUnifiedTopology: true });
  - const reference = await SomeReferencedClassModel.create({});
  + const reference = await SomeReferencedClassModel.create({ dummy: 'hello' });
  const { _id: id } = await SomeClassModel.create({ populateField: reference } as SomeClass);
  console.log(await SomeClassModel.findById(id).exec()); // output will be populated
  