Quote square brackets in shell commands.

This ensures compatibility with zsh.

Fix #2316.

docs/README.md

@@ -4,7 +4,7 @@ To generate the documentation, you first have to build it. Several packages are
 you can install them with the following command, at the root of the code repository:
 
 ```bash
-pip install -e .[docs]
+pip install -e ".[docs]"
 ```
 
 ## Packages installed