try to fix CI

Author: c-cube

.github/workflows/main.yml

@@ -21,7 +21,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2
-    - uses: ocaml/setup-ocaml@v2
+    - uses: ocaml/setup-ocaml@v3
       with:
         ocaml-compiler: ${{ matrix.ocaml-compiler }}
         dune-cache: true