Update swift-docc-plugin to fix a non-sendable warning (#12)

This PR resolves warnings:
```
…/.build/checkouts/swift-docc-plugin/Plugins/Swift-DocC Preview/Symbolic Links/SwiftDocCPluginUtilities/BuildGraph/DocumentationBuildGraph.swift:106:17: warning: capture of non-sendable type 'Target.Type' in an isolated closure
104 |             // Copy the closure and the target into a block operation object
105 |             let new = BlockOperation { [work, task] in
106 |                 work(task)
    |                 `- warning: capture of non-sendable type 'Target.Type' in an isolated closure
107 |             }
108 |             operationsByID[task.id] = new

…/.build/checkouts/swift-docc-plugin/Plugins/Swift-DocC Preview/Symbolic Links/SwiftDocCPluginUtilities/BuildGraph/DocumentationBuildGraph.swift:105:41: warning: capture of non-sendable type 'Target.Type' in an isolated closure
103 |             }
104 |             // Copy the closure and the target into a block operation object
105 |             let new = BlockOperation { [work, task] in
    |                                         `- warning: capture of non-sendable type 'Target.Type' in an isolated closure
106 |                 work(task)
107 |             }
```

Author: dkovba