Fix elided-named-lifetimes

[ehuss]

Rust has recently started warning about the missing lifetime here (via rust-lang/rust#129207).

warning: elided lifetime has a name
   --> src/data.rs:154:31
    |
151 |     pub(crate) fn subteams_of<'a>(
    |                               -- lifetime `'a` declared here
...
154 |     ) -> impl Iterator<Item = &Team> + 'a {
    |                               ^ this elided lifetime gets resolved as `'a`
    |
    = note: `#[warn(elided_named_lifetimes)]` on by default