Use correct token key name for lychee action

.github/workflows/check_links.yml

@@ -45,4 +45,4 @@ jobs:
           args: "--exclude-all-private --include-verbatim --max-concurrency 24 --require-https --verbose --no-progress --timeout 60 './docs/**/*.md' './docs/**/*.html' './imported_docs/**/*.md' './imported_docs/**/*.html' --exclude-path './docs/ehrql/includes/generated_docs'"
           fail: true
         env:
-          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}
+          token: ${{secrets.GITHUB_TOKEN}}