Quote "black[jupyter]" in README.md (#3007)

Author: dignissimus

README.md

@@ -39,7 +39,8 @@ Try it out now using the [Black Playground](https://black.vercel.app). Watch the
 ### Installation
 
 _Black_ can be installed by running `pip install black`. It requires Python 3.6.2+ to
-run. If you want to format Jupyter Notebooks, install with `pip install black[jupyter]`.
+run. If you want to format Jupyter Notebooks, install with
+`pip install 'black[jupyter]'`.
 
 If you can't wait for the latest _hotness_ and want to install from GitHub, use: