amending 1 documentation comment

I was hoping that `accept_options` could work with a `Virtus.value_object`, but it seems to be accepted only by `Virtus::Attribute` isn't it?
I tried to mixed them together and had an error like

```
ArgumentError: wrong number of arguments (2 for 0..1)
from ~/.rvm/gems/ruby-2.1.2@hsa/gems/virtus-1.0.3/lib/virtus/instance_methods.rb:16:in `initialize'
```

**Not related with this PR**
The following example doesn't work:

```ruby
class NoisyString < Virtus::Attribute
  def coerce(value)
    coercer[value.class].to_string.upcase
  end
end
```