Fix matbench benchmark.py

[naik-aakash]

Issue

Currently, when save folds is set to True in matbench_benchmark, it fails with the ind variable not defined error.

Fix

Use the fold_ind variable which is defined to fix the error

[naik-aakash]

Hi @ppdebreuck , can you please review this PR and merge it, if its fine with you as well 😄

[ppdebreuck]

Thanks @naik-aakash for spotting this ! I'll merge this, sorry for the latency !