Fix indexing in split commit messages
Hi ! this is expected and is coherent with other naming conventions in `datasets` such as parquet shards naming
When a large commit is split up, it seems the commit index in the message is zero-based while the total number is one-based. I came across this running `convert-to-parquet` and was wondering why there was no `6-of-6` commit. This PR fixes that by adding one to the commit index, so both are one-based.

Current behavior:
<img width="463" alt="Screenshot 2025-04-17 at 1 00 17 PM" src="https://github.com/user-attachments/assets/7f3d389e-cb92-405d-a3c2-f2b1cdf0cb79" />