Fix Windows path used for test_env in docs.
Fix Windows path used for test_env in docs.

Actually tested this one, as part of

```
test:vk_swiftshader --test_env="VK_ICD_FILENAMES=D:\\dev\\projects\\iree\\build-swiftshader\\Windows\\vk_swiftshader_icd.json"
```

with `--config=vk_swiftshader` passed to `bazel test`.

google/iree,google/iree,iree-org/iree,iree-org/iree,google/iree,google/iree,google/iree,google/iree,iree-org/iree,iree-org/iree,iree-org/iree,google/iree,iree-org/iree,iree-org/iree