fix the instruction issue (#731)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/731

the following command will run from the `executorch` root instead of `doc`
```
pip3 install -r ./.ci/docker/requirements-ci.txt
bash install_requirements.sh
```

Reviewed By: tarun292

Differential Revision: D49979270

fbshipit-source-id: e9170b6e0836ef0157e55e83a9b51ea0527ad451