Small correction in context.md

Just a really small correction ... in `context.md` there's this sentence. ``` A provider will attach an event listener to the DOM tree to catch the event, and if it will be able to satisfy the request MUST call stopPropagation on the event. ``` Should it be ... ``` A provider will attach an event listener to the DOM tree to catch the event, and if it will be able to satisfy the request it MUST call stopPropagation on the event. ``` So ... `it MUST...` ...