Fix Typo - r_object to rw_object

```
[error]: Unhandled exception in YARD::Handlers::Ruby::ActiveRecord::Fields::FieldHandler:
[error]:   in `db/schema.rb`:314:

	314: t.string   "status",        :limit => 10, :scale => 0

[error]: NameError: undefined local variable or method `r_object' for #<YARD::Handlers::Ruby::ActiveRecord::Fields::FieldHandler:0x000000068c36f0>
[error]: Stack trace:
	/home/ray/.rvm/gems/ruby-2.0.0-p195/bundler/gems/yard-activerecord-3814b8bc0721/lib/yard-activerecord/fields/field_handler.rb:31:in `block in process'
	/home/ray/.rvm/gems/ruby-2.0.0-p195/bundler/gems/yard-activerecord-3814b8bc0721/lib/yard-activerecord/fields/field_handler.rb:24:in `each'
	/home/ray/.rvm/gems/ruby-2.0.0-p195/bundler/gems/yard-activerecord-3814b8bc0721/lib/yard-activerecord/fields/field_handler.rb:24:in `process'
	/home/ray/.rvm/gems/ruby-2.0.0-p195/gems/yard-0.8.7/lib/yard/handlers/processor.rb:114:in `block (2 levels) in process'
	/home/ray/.rvm/gems/ruby-2.0.0-p195/gems/yard-0.8.7/lib/yard/handlers/processor.rb:112:in `each'
	/home/ray/.rvm/gems/ruby-2.0.0-p195/gems/yard-0.8.7/lib/yard/handlers/processor.rb:112:in `block in process'
```