Update it_IT.lang

Spelling corrections following https://github.com/Azanor/thaumcraft/pull/1329.
Also corrected a couple more issues.
Changed all the instances of "flux" from flusso back to "flux". This is to
a) make it easier for Italan asking for help to actually translate their requests
b) avoid confusion between "flusso" of air from bellow or essentia and "flux" meant as the bad effect
c) mantain constitency. Essentia is kept as such. Flux should be, too.

Also changed some words around on my own, trying to adhere to what i think it's the correct italian spelling, but i might be proven wrong (old dictionaries are old)