Fix Gibson link in readme (facebookresearch#666)

The link to the Gibson dataset was changed incorrectly

README.md

@@ -177,7 +177,7 @@ python -m habitat_sim.utils.datasets_download --uids replica_cad_dataset
 The full Matterport3D (MP3D) dataset for use with Habitat can be downloaded using the official [Matterport3D](https://niessner.github.io/Matterport/) download script as follows: `python download_mp.py --task habitat -o data/scene_datasets/mp3d/`. You only need the habitat zip archive and not the entire Matterport3D dataset. Note that this download script requires python 2.7 to run. Extract the matterport data to `data/scene_datasets/mp3d`.
 
 #### Gibson
-Download the Habitat related Gibson dataset following the instructions [here](https://aihabitat.org/datasets/replica_cad/). After downloading extract the dataset to folder `habitat-lab/data/scene_datasets/gibson/` folder (this folder should contain the `.glb` files from Gibson).
+Download the Habitat related Gibson dataset following the instructions [here](https://github.com/StanfordVL/GibsonEnv#database). After downloading extract the dataset to folder `habitat-lab/data/scene_datasets/gibson/` folder (this folder should contain the `.glb` files from Gibson).
 
 
 ### Task datasets