Change as_str → to_string in proc_macro::Ident::span() docs
Change as_str → to_string in proc_macro::Ident::span() docs

There is no `as_str` function on Ident any more.

Also change it to an intra doc link while we're at it.

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust