Added correct terminal command for zsh users

Author: Harsh-Goel-1

Contributing.md

@@ -23,9 +23,15 @@ git checkout -b feature/your-descriptive-name
 ## Installing from Source
 
 Install pgmpy (plus testing dependencies) in editable mode:
+
+For `bash` users:
 ```
 pip install -e .[tests]
 ```
+For `zsh` users:
+```
+pip install -e ".[tests]"
+```
 This lets you tweak code and immediately see your changes without re-installing.
 
 ## Running Tests