style: update download help text (#1135)

# What does this PR do?
[Provide a short summary of what this PR does and why. Link to relevant
issues if applicable.]

Based on the cade:

6b1773d530/llama_stack/cli/download.py (L454)
and the test, it can use comma to specify multiple model ids. So update
the usage.
```
$ llama model download --source meta --model-id Llama3.2-1B,Llama3.2-3B
Please provide the signed URL for model Llama3.2-1B you received via email after visiting https://www.llama.com/llama-downloads/ (e.g., https://llama3-1.llamameta.net/*?Policy...):

Downloading checklist.chk            ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━        100.0%  156/156 bytes   -  0:00:00
Downloading tokenizer.model          ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━        100.0%  2.2/2.2 MB      -  0:00:00
Downloading params.json              ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━        100.0%  220/220 bytes   -  0:00:00
Downloading consolidated.00.pth      ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━        100.0%  2.5/2.5 GB      -  0:00:00

Successfully downloaded model to /Users/xx/.llama/checkpoints/Llama3.2-1B

[Optionally] To run MD5 checksums, use the following command: llama model verify-download --model-id Llama3.2-1B
Please provide the signed URL for model Llama3.2-3B you received via email after visiting https://www.llama.com/llama-downloads/ (e.g., https://llama3-1.llamameta.net/*?Policy...):
Downloading checklist.chk            ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━        100.0%  156/156 bytes   -  0:00:00
Downloading tokenizer.model          ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━        100.0%  2.2/2.2 MB      -  0:00:00
Downloading params.json              ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━        100.0%  220/220 bytes   -  0:00:00
Downloading consolidated.00.pth      ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━        100.0%  6.4/6.4 GB      -  0:00:00

Successfully downloaded model to /Users/xx/.llama/checkpoints/Llama3.2-3B


$ llama model download --source huggingface --model-id Llama3.2-1B,Llama3.2-3B
original%2Fparams.json: 100%|██████████████████████████████████████████████████████████| 220/220 [00:00<00:00, 564kB/
Successfully downloaded model to /Users/xx/.llama/checkpoints/Llama3.2-1B
...
tokenizer.json: 100%|█████████████████████████████████████████████████████████████| 9.09M/9.09M [00:00<00:00, 9.18MB/s]
Successfully downloaded model to /Users/xxx/.llama/checkpoints/Llama3.2-3B


before:
$ llama model download --help
 --model-id MODEL_ID   See `llama model list` or `llama model list --show-all` for the list of available models

after:
$ llama model download --help
  --model-id MODEL_ID   See `llama model list` or `llama model list --show-all` for the list of available models. Specify multiple model IDs with commas, e.g. --model-id Llama3.2-1B,Llama3.2-3B
```

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
[Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.*]

[//]: # (## Documentation)

Signed-off-by: reidliu <reid201711@gmail.com>
Co-authored-by: reidliu <reid201711@gmail.com>

llama_stack/cli/download.py

@@ -56,7 +56,7 @@ def setup_download_parser(parser: argparse.ArgumentParser) -> None:
     parser.add_argument(
         "--model-id",
         required=False,
-        help="See `llama model list` or `llama model list --show-all` for the list of available models",
+        help="See `llama model list` or `llama model list --show-all` for the list of available models. Specify multiple model IDs with commas, e.g. --model-id Llama3.2-1B,Llama3.2-3B",
     )
     parser.add_argument(
         "--hf-token",