docs/example_project.md: Drop out-of-sync "cp env.example .env" (#10)

There is no file "env.example", and file ".env" already exists
in Git.

Follow-up to commit 80aac17.

Author: hartwork

docs/example_project.md

@@ -7,7 +7,6 @@ Run it locally and play with it to get a better understanding of how the library
 ```bash
 poetry install
 cd example
-cp env.example .env
 poetry run python manage.py migrate
 poetry run python manage.py runserver
 ```