Give "corrected" code in examples

[ChrisJefferson]

clippy recently pointed me to this: https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes

This tells me I should make use of "lifetime elision". I can see an example at the bottom, which does indeed show me something which looks like my code, but nothing seems to tell me how to make use of "lifetime elision". Perhaps every example should also come with a "fixed" version, so readers can see the intended outcome of the suggestion?

[flip1995]

I think, that most of the documentation comes with a good and bad example. But old lints (like this one) may only have a bad example, because back then, this was the standard way of writing lint documentation. So we still need to catch up.

[phansch]

Reopening because there's still other lints without 'fixed' versions.