Fix guide. sample code syntax error.[ci skip]

I read Active Record Validations' guide page. 
I execute sample code.
It raises syntax error.

~~~bash
SyntaxError: %sample_path%/computer.rb:4: syntax error, unexpected ':'
                    unless: Proc.new { |c| c.trackpad.present? }
~~~

There is no comma at tail of if-symbol line.