ci: update docs runner to zig v0.13.0

Author: rockorager

.github/workflows/docs.yml

@@ -27,7 +27,7 @@ jobs:
         uses: actions/configure-pages@v2
       - uses: goto-bus-stop/setup-zig@v2
         with:
-          version: 0.12.0
+          version: 0.13.0
       - run: zig build docs
       - name: Upload artifact
         uses: actions/upload-pages-artifact@v1