fix 80X instructions

[kpedro88]

An old, nonexistent branch name was referenced in the readme.

[alefisico]

the instructions do not refer to a branch, but to a tag:
https://github.com/cms-jet/JetToolbox/tree/jetToolbox_80X_V2
the branches I keep it for working branches, the tags for final unchange code.

[kpedro88]

I figured out the problem I was having: the system version of Git on the cluster I was using was too old to understand -b [tagname] in git clone. Using the CMSSW version of Git works.

However, the README is still out of date (V2 instead of V3), so please reopen this. I've updated my branch so it references the V3 tag.

[alefisico]

well, now that I remember I want to first merged #47 but it was a bit of a controversy with the pt cut in raw jets. (maybe you remember that you brought this issue with the differences in the weights, or something like that)
I think that PR should be included in 91X since has other tools that people are using them already

[kpedro88]

Indeed, I remember #47... personally I would still prefer the solution I originally proposed, applying the pT cut on corrected jets (even though it adds another producer/collection to the process, I think it's the best balance between CPU efficiency and physics needs).