Fix broken redirection (#36827)

.openpublishing.redirection.json

@@ -137,7 +137,7 @@
     },
     {
       "source_path": "docs/spark/index.yml",
-      "redirect_url": "/previous-versions/dotnet/spark",
+      "redirect_url": "/previous-versions/dotnet/spark/what-is-spark",
       "redirect_document_id": false
     },
     {