Update checkout version in README.md (#574)

README.md

@@ -7,7 +7,7 @@ A GitHub action to save / restore the coursier / sbt / mill / Ammonite caches of
 Add a `coursier/cache-action@v6` step to your YAML workflow, like
 ```yaml
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: coursier/cache-action@v6
 ```