docs: knot-hosting: specify running a linux distribution

Signed-off-by: glomdom <[e-mail]>

docs/knot-hosting.md

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 So you want to run your own knot server? Great! Here are a few prerequisites:
 
-1. A server of some kind (a VPS, a Raspberry Pi, etc.). Preferably running a Linux of some kind.
+1. A server of some kind (a VPS, a Raspberry Pi, etc.). Preferably running a Linux distribution of some kind.
 2. A (sub)domain name. People generally use `knot.example.com`.
 3. A valid SSL certificate for your domain.