fix build warning by making `options` immutable
fixes this warning:
```
warning: variable does not need to be mutable
--> src/main.rs:140:9
|
140 | let mut options = vec![
| ----^^^^^^^
| |
| help: remove this `mut`
|
= note: `#[warn(unused_mut)]` (part of `#[warn(unused)]`) on by default
```