Update ci.yml

Author: cristianoc

.github/workflows/ci.yml

@@ -62,7 +62,7 @@ jobs:
             build: opam exec -- dune build
           - os: ubuntu-latest
             target: ocaml.5.3
-            ocaml-compiler: 5.3.x
+            ocaml-compiler: 5.3.0~rc1
             build: opam exec -- dune build
 
     runs-on: ${{matrix.os}}