Update README.md

[saforem2]

Copilot Summary

This pull request updates the ALCF/README.md file to reflect changes in the main training script used for distributed training on ALCF systems. The previous script train_llama_alcf.sh has been replaced with train_aGPT_7B.sh throughout the documentation.

Key changes include:

• Updated the main entry point for launching distributed training from train_llama_alcf.sh to train_aGPT_7B.sh.
• Changed references to the script in the instructions for submitting jobs and setting up the environment. [1] [2] [3]
• Modified example commands and code snippets to use train_aGPT_7B.sh instead of train_llama_alcf.sh. [1] [2] [3] [4] [5] [6]