Fix broken image in contributing.md file

[TimOsahenru]

The file path for the broken image is incorrect, get the correct path to display the correct image

Implementation details

Change the file path in the CONTRIBUTING.md file from ![Black Python Devs Codespace](/assets/images/BlackPythonDevs_codespace.png) to ![Black Python Devs Codespace](/assets/images/blackpythondevs_codespace.png) and be sure the image appears correctly

[GaurangMapuskar]

@TimOsahenru I will pick it up