README.md: Use checkout@v6 in example code

Author: gruenich

README.md

@@ -6,7 +6,7 @@ Any warnings or errors will be annotated in the Pull Request.
 ## Usage
 
 ```yml
-- uses: actions/checkout@v5
+- uses: actions/checkout@v6
 - uses: codespell-project/actions-codespell@v2
 ```