README: bump version of actions/checkout to v4

Author: krzema12

README.md

@@ -67,7 +67,7 @@ jobs:
   validate-typings:
     runs-on: "ubuntu-latest"
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: typesafegithub/github-actions-typing@v1
 ```