Docs: Update block-filters.md

[jakeparis]

Clarify the parameters for the blocks.getSaveElement callback

Description

I've clarified the parameters in the blocks.getSaveElement filter callback.

Types of changes

Documentation

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).
• I've updated related schemas if appropriate.

[github-actions]

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[gziolo]

Thank you.