Fix Typo in The Future Trait Chapter

[sunrosa]

I don't know what style guide the book follows, but it also works when capitalized. &mut self is short for self: &mut Self. Self is capitalized as a type, and lower case as a receiver identifier. Since two receiver types are being compared, it also makes sense for Self to be capitalized as in its de-sugared form.

[Kartik1397]

Since two receiver types are being compared, it also makes sense for Self to be capitalized as in its de-sugared form.

Right, in the context of types &mut Self seems correct.

Thanks for the review @sunrosa!