fix shfmt action

Author: angristan

.github/workflows/lint.yml

@@ -16,4 +16,4 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: mfinelli/setup-shfmt@v3
-      - run: shfmt -d wireguard-install.bash
+      - run: shfmt -d wireguard-install.sh