fix baselines pip installation instruction (#35)

README.md

@@ -79,8 +79,9 @@ install the source as follows:
 ```commandline
 git clone [email protected]:corl-team/xland-minigrid.git
 cd xland-minigrid
+
 # additional dependencies for baselines
-pip install -e ".[dev,benchmark]"
+pip install -e ".[dev,baselines]"
 ```
 Note that the installation of JAX may differ depending on your hardware accelerator! 
 We advise users to explicitly install the correct JAX version (see the [official installation guide](https://github.com/google/jax#installation)).