Update manual update section to mention deleting core mods

[itscynxx]

Closes #266

The main issue is actually likely going to be people not linking to this page and just spreading word around of how to manually update. I'll update the video tag we use in the Discord server once I clean things up

Not sure how effective this is either, it may be worth doing something similar to what I do in VanillaPlus, and instead of having a separate section for if you're updating, just tell the user

If you're updating from an older version, remove Northstar.Client

(this is in terms of VanillaPlus)

This would be reworded to something like

If you're updating Northstar, delete Northstar.Client, Northstar.Custom, and Northstar.CustomServers from Titanfall2/R2Northstar/mods

Would like feedback on this version vs a version where we just say to remove the core mods in the install instructions, and possibly just remove the updating section

[GeckoEidechse]

Comments from previous reviews have been addressed. PR implements and closes #266