ci: tweak 5.3 OCaml compiler version

Author: andreypopp

.github/workflows/main.yml

@@ -21,7 +21,7 @@ jobs:
           - "4.14"
           - "5.1"
           - "5.2"
-          - "5.3"
+          - "5.3.0~beta2"
 
     runs-on: ${{ matrix.os }}