Update proveWithdrawal.md

[danielsimao]

PR-Codex overview

This PR introduces new functions getL2Output and getWithdrawals to handle withdrawal operations in the proveWithdrawal action.

Detailed summary

• Added imports for getL2Output and getWithdrawals
• Replaced usage of walletClientL1.getL2Output with getL2Output
• Updated parameters in client.proveWithdrawal call

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