gh-pages: update CUSTOM_DOMAIN env variable

docs.refine.rs -> refine.rs

.github/workflows/docs.yml

@@ -17,5 +17,5 @@ jobs:
         uses: mhausenblas/mkdocs-deploy-gh-pages@master
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-          CUSTOM_DOMAIN: docs.refine.rs
+          CUSTOM_DOMAIN: refine.rs
           REQUIREMENTS: ./requirements.docs.txt