crypto/pbkdf2: add missing error return value in example

[bystones]

This function returns two values, one was missing in the example.

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Patch Set 1: Code-Review+2

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Patch Set 1: Auto-Submit+1 Code-Review+1 Commit-Queue+1

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Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-10-23T20:55:21Z","revision":"877e93a034a1d7157035167a2314e863b99ff43c"}

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Patch Set 1: Code-Review+2

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Patch Set 1: -Commit-Queue

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