Update developer_docs.md (#129)

Update instructions to install cookiecutter instance in developer mode.

Author: Philipp Weiler

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

@@ -193,7 +193,8 @@ In addition to the packages needed to _use_ this package, you need additional py
 the documentation_. It's easy to install them using `pip`:
 
 ```bash
-pip install "{{ cookiecutter.project_name }}[dev,test,doc]"
+cd {{ cookiecutter.project_name }}
+pip install -e ".[dev,test,doc]"
 ```
 
 ### Code-style