Replace outdated custom doc parameter

[finestructure]

This parameter isn't valid anymore and it seems some folks are copying it from our docs:

Error while generating docs: retryLimitExceeded(lastError: Optional("Shell command failed:\n    env DEVELOPER_DIR=/Applications/Xcode-15.4.0.app xcrun swift package --allow-writing-to-directory .docs/sbooth/sfbaudioengine/main generate-documentation --emit-digest --disable-indexing --output-path .docs/sbooth/sfbaudioengine/main --hosting-base-path sbooth/sfbaudioengine/main --source-service github --source-service-base-url https://github.com/sbooth/SFBAudioEngine/blob/main --checkout-path $workDir --target CSFBAudioEngine --include-extended-types"))

Better replace it with something that doesn't look valid.