Broken section links in environment variables generated documentation

On this page:
* https://intel.github.io/llvm-docs/EnvironmentVariables.html

The "See below" links in the environment variables table are broken, it seems that sphinx is turning the `_` into `-` when generating the title anchors, see:

This link goes to the correct location:
* https://intel.github.io/llvm-docs/EnvironmentVariables.html#oneapi-device-selector

This link doesn't:
* https://intel.github.io/llvm-docs/EnvironmentVariables.html#oneapi_device_selector

But the second one is the anchor that is used in the table for the "See below" links.

In the github rendered version the generated title anchors don't modify the `_`, so the links are working properly, see:

* https://github.com/intel/llvm/blob/sycl/sycl/doc/EnvironmentVariables.md#oneapi_device_selector