fix: URL to install DensePose thorugh pip
Summary:
DensePose Pip install URL points to  master branch which no longer exists. Fix url to install from main branch instead
Pull Request resolved:
@@ -70,7 +70,7 @@ The following dependencies are needed:
DensePose can then be installed from this repository with:
```
-pip install git+https://github.com/facebookresearch/detectron2@master#subdirectory=projects/DensePose
+pip install git+https://github.com/facebookresearch/detectron2@main#subdirectory=projects/DensePose
```
After installation, the package will be importable as `densepose`.
