getting 404 error while developing an example module
The indentation of your hello.routing.yml file is wrong. It should be as the following one. (See user.routing.yml.)

user.pass:
  path: '/user/password'
  defaults:
    _form: '\Drupal\user\Form\UserPasswordForm'
    _title: 'Reset your password'
  requirements:
    _access: 'TRUE'
  options:
    _maintenance_access: TRUE


Indentation in YAML files is very important, since it changes the file content.

Notice that you can use _access: 'TRUE', when the page doesn't require a specific permission. The _maintenance_access: TRUE line is not necessary. If you use it, users have access to your route even when the site is in maintenance mode; other requirements are still respected, so you could have something similar to the following.

user.login:
  path: '/user/login'
  defaults:
    _form: '\Drupal\user\Form\UserLoginForm'
    _title: 'Log in'
  requirements:
    _user_is_logged_in: 'FALSE'
  options:
    _maintenance_access: TRUE


Before making any changes, uninstall the module, fix the file, and re-install the module.

As side note, you don't need the use Drupal\Core\Routing\RouteMatchInterface; line in your hello.module file, since you aren't using that PHP interface.