Update ch04-02-references-and-borrowing.md

[mayur-padshala]

It is assumed that the change function will mutate the borrowed reference. It depends on the implementation of the function whether it will or will not mutate. It is more appropriate to use the word "can" instead of "will".

[chriskrycho]

Thanks for the PR. In context, however, the point is that the function is mutating. Also, as a rule, Rust functions do not take parameters mutably unless they are going to mutate. I am closing this and leaving it as is accordingly.