nixos/stirling-pdf: fix documentation link formatting

Changed files
+1 -1
nixos
modules
services
web-apps
+1 -1
nixos/modules/services/web-apps/stirling-pdf.nix
···
};
description = ''
Environment variables for the stirling-pdf app.
-
See https://github.com/Stirling-Tools/Stirling-PDF#customisation for available options.
+
See <https://github.com/Stirling-Tools/Stirling-PDF#customisation> for available options.
'';
};