Add base-url to swift-doc generate action

Author: slashmo

.github/workflows/documentation.yaml

@@ -14,6 +14,7 @@ jobs:
         uses: SwiftDocOrg/swift-doc@master
         with:
           format: html
+          base-url: /swift-w3c-trace-context/
           module-name: "W3C Trace Context"
           output: Documentation
       - name: Update Permissions