Address following error from rustdoc tests:

Address following error from rustdoc tests:

error[E0106]: missing lifetime specifier
 --> /checkout/obj/build/x86_64-unknown-linux-gnu/test/error-index.md:11424:23
  |
9 | fn demo(s: &mut S) -> &mut String { let p = &mut *(*s).data; p }
  |                       ^ expected lifetime parameter
  |
  = help: this function's return type contains a borrowed value, but the signature does not say which one of `s`'s 2 lifetimes it is borrowed from