Update Downstream.yml

Author: ChrisRackauckas

.github/workflows/Downstream.yml

@@ -26,6 +26,7 @@ jobs:
         os: [ubuntu-latest]
         package:
           - {user: SciML, repo: SciMLBase.jl, group: Downstream}
+          - {user: SciML, repo: SciMLBase.jl, group: SymbolicIndexingInterface}
           - {user: SciML, repo: Catalyst.jl, group: All}
           - {user: SciML, repo: CellMLToolkit.jl, group: Core}
           - {user: SciML, repo: SBMLToolkit.jl, group: All}