Fix up link to skylark documentation.
Closes
@@ -36,7 +36,7 @@ Build and test software of any size, quickly and reliably.
* [Bazel command line](https://docs.bazel.build/user-manual.html)
* [Rule reference](https://docs.bazel.build/be/overview.html)
* [Use the query command](https://docs.bazel.build/query.html)
- * [Extend Bazel](https://docs.bazel.build/skylark/index.html)
+ * [Extend Bazel](https://docs.bazel.build/skylark/concepts.html)
* [Write tests](https://docs.bazel.build/test-encyclopedia.html)
* [Roadmap](https://bazel.build/roadmap.html)
* [Who is using Bazel?](https://github.com/bazelbuild/bazel/wiki/Bazel-Users)
