PUBs broadcasting rules, empty lines in code block

[GuillermoAbadLopez]

Tackles Issue: #4371

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Pull request overview

This PR removes two unnecessary blank lines from a code cell in the primitive input-output guide, improving code formatting consistency. The change addresses Issue #4371 regarding PUBs broadcasting rules and empty lines in code blocks.

• Removes two consecutive blank lines after the "# Broadcast single observable" comment
• Improves consistency with other example separators in the same code block

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[abbycross]

Thanks!