Let find_package(executorch) find the correct include directory (#6102)

Summary:
There's a typo in `executorch-wheel-config.cmake` that points to the wrong `include` path:
```
<site-packages>/executorch/share/cmake/include
```

Where it actually should be
```
<site-packages>/executorch/include
```

Fixing this issue. Verified it on [build_torchao_ops.sh](https://github.com/pytorch/ao/blob/main/torchao/experimental/build_torchao_ops.sh)

Pull Request resolved: https://github.com/pytorch/executorch/pull/6102

Reviewed By: lucylq

Differential Revision: D64189337

Pulled By: larryliu0820

fbshipit-source-id: 13033587f5499537623995b8f9457fb47d780340