Beta 3 --> Beta 4: "Routes use _controller instead of _content"

When defining routes in the .routing.yml file, the _content property is no longer valid. I got a "page not found" error when browsing to such a route.

Use _controller instead.

Old:

optimizely.add_update.oid:
  path: /admin/config/system/optimizely/add_update/{oid}
  defaults:
    _content: \Drupal\optimizely\DoUpdate::buildUpdateForm
    _title: Optimizely Edit Project
  requirements:
    _permission: administer optimizely

New:

optimizely.add_update.oid:
  path: /admin/config/system/optimizely/add_update/{oid}
  defaults:
    _controller: \Drupal\optimizely\DoUpdate::buildUpdateForm
    _title: Optimizely Edit Project
  requirements:
    _permission: administer optimizely


Source:

Routes use _controller instead of _content
https://www.drupal.org/node/2378809