Change MultiMC to Prism Launcher

[mazunki]

Since MultiMC has been largely abandoned by the community due to some security vulnerabilities involving a disrespectful and condescending developer, and the community mostly using Prism Launcher anyway, we may as well just update our references to the launcher in the tutorial.

[parov0z]

I think the right way would be to say "MultiMC and its forks", and do not speak for an entire community. If things will go wrong again, should we change launcher name here every time?

[RunningDroid]

@parov0z Prism Launcher appears to be the only active fork (aside from cracked or offline forks), do you know of others?

[redmine4404]

I never heard of a MultiMC big drama, it's more the polyMC side who have some issue regarding developpers, MultiMC is yes not very active but it's the base of MultiMC/PolyMc and i think it's better to say "MultiMC and its deriative" for less know fork.