nomicon: reference to copy_lifetime

[durka]

http://doc.rust-lang.org/nightly/nomicon/unbounded-lifetimes.html mentions a fn called copy_lifetime but there is no such fn (anymore?).

[apasel422]

CC @gankro.

[Gankra]

Yeah this is what it was: https://doc.rust-lang.org/1.0.0/std/mem/fn.copy_lifetime.html

text needs updating.