fix: typo in log message

[leopardracer]

Description:
This pull request corrects a typo in the log message within the crates/proposer-client/src/rpc/mod.rs file. The word "fullfilled" has been changed to the correct spelling "fulfilled" in the info log statement for mock proof requests. No functional changes were made; this is a minor improvement for clarity and professionalism in log output.

[Ekleog-Polygon]

LGTM, thank you! Unfortunately we are still waiting for legal approval before landing external contributors' PRs, but the changes here are technically ready to merge 😄

[Ekleog-Polygon]

Oh, just one thing: to get CI to pass you'll need to adjust your commits and PR title to eg. fix: typo in log message

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ atanmarko
❌ leopardracer
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