There is no longer any need to use this extension, since the functionality is now provided natively in Visual Studio Code.
This can configured in Keyboard Shortcuts under File: Copy Relative Path of Active File (copyRelativeFilePath). It is recommended that you remove the When condition of !editorFocus to make the command usable when you're actually editing the file.
A Visual Studio Code extension for copying the relative path of an active file to the clipboard.
Add something like this to your keybindings.json
{"key": "cmd+shift+c", "command": "extension.copyRelativePathOfActiveFile"}