docs: fix typo in snippets_test.py

[mataralhawiti]

Fix typo in docs related to 9.5.2. Composition with AfterWatermark
https://beam.apache.org/documentation/programming-guide/#composite-afterwatermark

addresses #34224

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[mataralhawiti]

assign set of reviewers

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[mataralhawiti]

@liferoad I'd appreciate any advice or guidance on this, thanks.