Update pip install instructions to work for Windows and zsh user (#9346)

Co-authored-by: Thibaud Colas <thibaudcolas@gmail.com>

CONTRIBUTORS.rst

@@ -647,6 +647,7 @@ Contributors
 * Mary Ayobami
 * Bolarinwa Comfort Ajayi
 * Mary Ojo
+* Oluwaloseyi Adeleye
 
 Translators
 ===========

docs/contributing/developing.md

@@ -29,7 +29,7 @@ cd wagtail
 **With your preferred virtualenv activated,** install the Wagtail package in development mode with the included testing and documentation dependencies:
 
 ```sh
-pip install -e '.[testing,docs]' -U
+pip install -e ."[testing,docs]" -U
 ```
 
 Install the tool chain for building static assets: