fix: std publish directory (FuelLabs#4590)

## Description
Updates to `forc doc` changed the output structure of generated docs.
This updates the publish directory so that mdbook can find the
`index.html`.

.github/workflows/gh-pages.yml

@@ -120,7 +120,7 @@ jobs:
         uses: peaceiris/actions-gh-pages@v3
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
-          publish_dir: ./sway-lib-std/out/doc
+          publish_dir: ./sway-lib-std/out/doc/std
           destination_dir: ${{ steps.branch_name.outputs.BRANCH_NAME }}/std
         if: startsWith(github.ref, 'refs/tags')