Fix typo in CONTRIBUTING.md (robustness-gym#48)

CONTRIBUTING.md

@@ -87,10 +87,10 @@ Set up a development environment with `poetry` by running the following command
    $ pip install poetry
    $ poetry install
    ```
-Note: in order to pass the full test suite (step 5), you'll need to install all extra in addition. 
-```bash
-   $ poetry install --extras "adversarial augmentation summarization text vision"
-```
+   Note: in order to pass the full test suite (step 5), you'll need to install all extras using the following command: 
+   ```bash
+      $ poetry install --extras "adversarial augmentation summarization text vision"
+   ```
 5. Develop features on your branch.
 
    As you work on the features, you should make sure that the test suite
@@ -214,4 +214,4 @@ $ python -m unittest discover -s tests -t . -v
 ### Style guide
 
 For documentation strings, `robustnessgym` follows the 
-[google style](https://google.github.io/styleguide/pyguide.html).
+[google style](https://google.github.io/styleguide/pyguide.html).