chore: rename task_config to benchmark_config (#1397)

# What does this PR do?

- This was missed from previous deprecation:
https://github.com/meta-llama/llama-stack/pull/1186
- Part of https://github.com/meta-llama/llama-stack/issues/1396

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

## Test Plan
```
pytest -v -s --nbval-lax ./llama-stack/docs/notebooks/Llama_Stack_Benchmark_Evals.ipynb 
```

[//]: # (## Documentation)

docs/source/building_applications/evaluation.md

@@ -19,7 +19,7 @@ response = client.benchmarks.register(
 # Run evaluation
 job = client.eval.run_eval(
     benchmark_id="my_eval",
-    task_config={
+    benchmark_config={
         "type": "app",
         "eval_candidate": {"type": "agent", "config": agent_config},
     },