docs: change compositionality to composability
Updated CONTRIBUTING.md for 2014
auto merge of #13939 : richo/rust/docs/composability, r=thestinger

While there are various references to the work compositionality on the web, I can't find any reference to it being an actual word. My understanding is that composability is what's actually meant here anyway.
auto merge of #13952 : aaronraimist/rust/patch-1, r=thestinger