docs: Spelling error fix on fuzzing.md

Summary:
Backport of core PR15185:
https://github.com/bitcoin/bitcoin/pull/15185/commits

Depends on D4613.

Test Plan: Read the doc.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4614