Piglins barter nether brick blocks instead of nether brick item

Summary:
When bartering with piglins, sometimes they sell nether brick blocks.

Steps to Reproduce:

1. Barter with piglins

Observed Results:
Piglins sometimes give nether brick blocks.

Expected Results:
Piglins should sometimes give the nether brick item.

Fix:
Remove the underscore in "nether_brick" in the loot table of piglin_barter.json