Update installation for generative example (#1399)

Fixes # .

### Description
Update installation for generative example

### Checks
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [ ] Avoid including large-size files in the PR.
- [ ] Clean up long text outputs from code cells in the notebook.
- [ ] For security purposes, please check the contents and remove any
sensitive info such as user names and private key.
- [ ] Ensure (1) hyperlinks and markdown anchors are working (2) use
relative paths for tutorial repo files (3) put figure and graphs in the
`./figure` folder
- [ ] Notebook runs automatically `./runner.sh -t <path to .ipynb file>`

---------

Signed-off-by: Can-Zhao <[email protected]>

Author: Can-Zhao

generative/README.md

@@ -6,16 +6,7 @@ The examples are designed as demonstration to showcase the training process for
 ## Installation
 ```
 pip install lpips
-pip install git+https://github.com/Project-MONAI/[email protected]
-```
-
-Or install it from source:
-```
-pip install lpips
-git clone https://github.com/Project-MONAI/GenerativeModels.git
-cd GenerativeModels/
-python setup.py install
-cd ..
+pip install monai-generative==0.2.2
 ```
 
 ## [Brats 3D latent diffusion model](./3d_ldm/README.md)