Fix package manifest

[giginet]

Bug/issue #, if applicable:

Summary

In current master (c570b58), this package uses swift-argument parser ~> 1.0.1.
https://github.com/apple/swift-markdown/blob/c570b5873782f1a1cca5f649ebf1e0bc599bba51/Package.swift#L56

However, Swift 5.5 still uses 0.4.4.
https://github.com/apple/swift-markdown/blob/c570b5873782f1a1cca5f649ebf1e0bc599bba51/Package%40swift-5.5.swift#L58

Because of this, swift-docc fails to resolve dependencies when it is built as other package's dependencies.

Dependencies

Testing

Run tests on Swift 5.5.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

Thanks for finding and fixing this.