Fix rustdoc's formatting of lifetimes

rustdoc was previously formatting lifetimes with two apostrophes, presumably as a result of #14797.
(of course, the link I just posted may not be a bug at all, but rather just a different choice of representation invariant that leads to the use of `"static"` above.  I assume `rustdoc` would not have the same hygiene issues that `rustc` has.)