docs(pipeline): fix num_workers docstring default from 8 to 0

[Anai-Guo]

What does this PR do?

Fixes #45557.

The docstring for num_workers in build_pipeline_init_args() states defaults to 8, but the actual runtime fallback in Pipeline.__call__() is 0:

# Line 1210 in base.py
if self._num_workers is None:
    num_workers = 0  # actual default

This PR updates the docstring to accurately reflect the runtime behavior (defaults to 0), preventing confusion for users relying on the docs for performance tuning.

Before submitting

• This PR fixes a typo or improves the docs (and does not need the community review, just direct merge by the author)
• Did you read the contributor guideline?

Fixes: #45557

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