Show correct routes prefix with namespace

When I write config/routes.rb like this,
```ruby
namespace :foo do
  match '/a',       to: 'a#index',  via: 'get'
  match '/a/:id',   to: 'a#show',   via: 'get'
  match '/b',       to: 'b#index',  via: 'get'
  match '/b/:id',   to: 'b#show',   via: 'get'
end
```
`rake routes` command shows as bellow.
```
Prefix Verb URI Pattern          Controller#Action
 foo_a GET  /foo/a(.:format)     foo/a#index
   foo GET  /foo/a/:id(.:format) foo/a#show
 foo_b GET  /foo/b(.:format)     foo/b#index
       GET  /foo/b/:id(.:format) foo/b#show
```
I think it should be displayed like this.
```
Prefix Verb URI Pattern          Controller#Action
 foo_a GET  /foo/a(.:format)     foo/a#index
       GET  /foo/a/:id(.:format) foo/a#show
 foo_b GET  /foo/b(.:format)     foo/b#index
       GET  /foo/b/:id(.:format) foo/b#show
```

So I fixed it.