[Doc improvement] Change the model download command  from 7b to 8b

https://docs.redhat.com/en/documentation/red_hat_enterprise_linux_ai/1.3/html/building_your_rhel_ai_environment/downloading_ad_models#downloading_models

Change granite-7b-starter to granite-8b-starter

Change From

ilab model download --repository docker://registry.redhat.io/rhelai1/granite-7b-starter --release latest

To

ilab model download --repository docker://registry.redhat.io/rhelai1/granite-8b-starter-v1 --release latest

To Reproduce Steps to reproduce the behavior:

Expected behavior

• <your text here>

Screenshots

• Attached Image 

Device Info (please complete the following information):

• Hardware Specs: [e.g. Apple M2 Pro Chip, 16 GB Memory, etc.]
• OS Version: [e.g. Mac OS 14.4.1, Fedora Linux 40]
• Python Version: [output of \\{{{}python --version{}}}]
• InstructLab Version: [output of \\{{{}ilab --version{}}}]

Additional context

• <your text here>
• …