docs: rust: general-information: simplify Kconfig example

There is no need to use `def_bool y if <expr>` -- one can simply write
`def_bool <expr>`.

In fact, the simpler form is how we actually use them in practice in
`init/Kconfig`.

Thus simplify the example.

Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260405235309.418950-30-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>