Fix typo in contributions section

[janie177]

Contributions instead of Contibutions.

[hannes-vernooij]

nice find

[MarijnS95]

error: writing `&mut Vec` instead of `&mut [_]` involves a new object where a slice will do
Error:    --> src/wrapper.rs:1[9](https://github.com/Traverse-Research/hassle-rs/runs/6155503861?check_suite_focus=true#step:7:9)8:22
    |
198 |         dxc_defines: &mut Vec<DxcDefine>,
    |                      ^^^^^^^^^^^^^^^^^^^ help: change this to: `&mut [DxcDefine]`
    |
    = note: `-D clippy::ptr-arg` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg

This looks to be a false-positive of Rust 1.60 and is gone in the next beta, as we call Vec::push on dxc_defines and you can't call that on (mutable) slices...

[MarijnS95]

@janie177 Let's go get this in after #41 :)