Clarify the instructions for setting up dev environment [skip ci] (#94155)
The `requirement.txt` file is in the PyTorch directory. The instructions to `clone` and `cd` to the PyTorch directory are in the later section under Get the PyTorch Source. So, the instructions as such gives an error that requirement.txt is not found.
```ERROR: Could not open requirements file: .. No such file or directory: 'requirements.txt' ```
This PR clarifies the usage of the command.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94155
Approved by: https://github.com/malfet
