fix URL

Author: vchuravy

.github/workflows/CacheNotebooks.yml

@@ -27,7 +27,7 @@ jobs:
         run: |
             git fetch origin
             git checkout tutorial_notebooks_new
-            curl https://raw.githubusercontent.com/trixi-framework/Trixi.jl/docs/Project.toml --output Project.toml
+            curl https://raw.githubusercontent.com/trixi-framework/Trixi.jl/refs/heads/main/docs/Project.toml --output Project.toml
             sed -i '1 a Trixi = "a7f1ee26-1774-49b1-8366-f1abc58fbfcb"' Project.toml
             cat Project.toml
             # `stable` is a symlink, so we need to retrieve it first to resolve it locally