README.md: fix settings filename in the development instructions

[RisingOpsNinja]

Building the backend fails, if you follow the development instructions in the readme:

[2024-08-29 08:24:54] [simple_wkd] DEBUG: Starting server...
[2024-08-29 08:24:54] [simple_wkd::settings] DEBUG: Parsing settings...
[2024-08-29 08:24:54] [simple_wkd::settings] ERROR: Unable to access settings file!
thread 'main' panicked at src/settings.rs:52:13:
Unable to access settings file!

The settings file is expected in backend/config.toml, but in the instructions it is copied to backend/example.toml:

$ cp example.config.toml backend/example.toml