Fix most likely copy/paste error

this lambda yielding a bool for `belongs_to` might confuse readers or conflate the :if block

which would probably look something like this 

```ruby
belongs_to :primary_agent, if: proc { |movie, params| params[:current_user].present? } do |movie, params|
    # in here, params is a hash containing the `:current_user` key
    params[:current_user]
  end
```

README.md

@@ -462,7 +462,7 @@ class MovieSerializer
 
   belongs_to :primary_agent do |movie, params|
     # in here, params is a hash containing the `:current_user` key
-    params[:current_user].is_employee? ? true : false
+    params[:current_user]
   end
 end