Fix docs typo for `ruff format` preview configuration (#8611)
## Summary
The ruff configuration section is called "format", rather than
"preview". Using the configuration as it was written in the docs gives
an error:
```
$ ruff format --check .
ruff failed
Cause: TOML parse error at line 143, column 1
|
143 | [tool.ruff.preview]
| ^^^^^^^^^^^^^^^^^^^
invalid type: map, expected a boolean
```
## Test Plan
Tested running `ruff format` with the following in my `pyproject.toml`:
```toml
[tool.ruff.format]
preview = true
```
and it worked properly (using preview rules for formatting).
