README fixed & small refactor for code.rs

This PR does some small fixes for the README and a small refactor for code.rs:

fix readme "Usage" step 4 indentation
fix readme "await" usage on non-async example
fix test custom_model_and_schema_path using the wrong output path
refactor code.rs::generate_for_table to directly push to a string buffer instead of separate
(side-effect) refactor code.rs to always only have one last ending new-line

Wulf / dsync