Missing minor installation instructions in README

[tollef]

Under the Installation for development instructions (https://github.com/hplt-project/OpusCleaner#installation-for-development), the instructions read:

opuscleaner-server --reload

But should include the commands:

  serve      run webserver
  sample     sample all datasets

So a better example would probably be to update it to:

opuscleaner-server serve --reload

The interface also loads up at localhost port 8000 for me, at the /frontend endpoint, not on port 5173 as is listed.