Fix wrong flag passed to swift-docc-plugin

[Kyle-Ye]

Fix wrong flag passed to swift-docc-plugin

See detail in #104 (comment)

Tested it locally this time via the same command SPI is using.

xcrun swift package --allow-writing-to-directory .docs/apple/swift-mmio/main generate-documentation --emit-digest --disable-indexing --output-path .docs/apple/swift-mmio/main --hosting-base-path apple/swift-mmio/main --source-service github --source-service-base-url https://github.com/apple/swift-mmio/blob/main --checkout-path $PWD --target MMIO --symbol-graph-minimum-access-level public

This time Error: Unknown option '--minimum-access-level public' would be solved.

But I still get the following same error on SPI build. Anyway this seems to be SPI issue and I have report it to upstream via SwiftPackageIndex/SwiftPackageIndex-Server#3343.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary