Fix installation instructions on contributing guide (#271)

{{cookiecutter.project_name}}/docs/contributing.md

@@ -54,7 +54,7 @@ and [prettier][prettier-editors].
 ## Writing tests
 
 ```{note}
-Remember to first install the package with `pip install '-e[dev,test]'`
+Remember to first install the package with `pip install -e '.[dev,test]'`
 ```
 
 This package uses the [pytest][] for automated testing. Please [write tests][scanpy-test-docs] for every function added