Use install in favour of deprecated develop command (#650)

`develop` has been deprecated in [this commit][1], `install` will
automatically use development mode.

[1]: 48ed586b

docs/docs/contributing.md

@@ -97,7 +97,7 @@ Now, you will need to install the required dependency for Poetry and be sure tha
 tests are passing on your machine:
 
 ```bash
-$ poetry develop
+$ poetry install
 $ poetry run pytest tests/
 ```