Update add-platform-label.yml (#345)

Author: kahest

.github/workflows/add-platform-label.yml

@@ -13,5 +13,5 @@ jobs:
     steps:
       - uses: andymckay/labeler@e6c4322d0397f3240f0e7e30a33b5c5df2d39e90 #1.0.4
         with:
-          add-labels: "Platform: Dart"
+          add-labels: "Dart"
           repo-token: ${{ secrets.GITHUB_TOKEN }}