Update SERVER_GUIDE.txt

[OpusC]

remove section on pre-generating the world as the Chunk-Pregenerator (by Speiger) mod was removed in release 1.84

[Kanzaji]

Just a quick note, please target the develop branch with this PR 😄

[NielsPilgaard]

Thank you for your contribution 😊 this is fine to go directly to master

[Kanzaji]

PR Into the master branch 👀 That's something new 😅

[Kanzaji]

Ah alrighty 😄 Yea actually makes sense why now that I think about it.

[NielsPilgaard]

Since it's a README update I'd rather have it out now than on next release.