Fix lifetime description in a function signature

[netaneld122]

I think that what the author meant here is that the returned reference can't outlive lifetime 'a (i.e will live at most as long as lifetime 'a and not at least as long as lifetime 'a).

[chriskrycho]

Thanks for the suggestion, but actually the text is correct—this is a common confusion, and one I had very early on as well. You may find my comment in an identical PR opened a while ago helpful!