Change master to main in densepose's pip install.

## 📚 Documentation Issue

This issue category is for problems about existing documentation, not for asking how-to questions.

* Provide a link to an existing documentation/comment/tutorial:
https://github.com/facebookresearch/detectron2/tree/main/projects/DensePose

* How should the above documentation/comment/tutorial improve:
The part where pip install of denspose is given it should be changed to

```bash
pip install git+https://github.com/facebookresearch/detectron2@main#subdirectory=projects/DensePose
```
as the `master` tag of github's main repo is changed to `main`
Also i couldn't find any docs for using the densepose packge from pip.

Start at the linked projects/DensePose documentation and locate the DensePose pip-install command. Update the repository reference from master to main, then verify the command and clarify whether the requested pip package usage documentation is also in scope.

Written by the indexing model from the issue text.