Update companion-installation.md

[Kirk1984]

This little missing "-" took me waaaay tot loon to figure out ;)

[unixfox]

There is no missing "-". It's indeed:

invidious_companion:
  - private_url: "http://localhost:8282"
    public_url: "http://localhost:8282"

And not:

invidious_companion:
  - private_url: "http://localhost:8282"
  - public_url: "http://localhost:8282"

[Kirk1984]

Ah ok, i see now. My problem was another config option. They just got "fixed" at the same time by me. Sorry.