Fix explicit_iter_loop pedantic lint 🎨

[nicokosi]

Lint warning, before this change:

$ cargo clippy

140 |     for (author, events) in events_per_author.iter() {
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^ help: to write this more concisely, try: `events_per_author`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#explicit_iter_loop
    = note: `-W clippy::explicit-iter-loop` implied by `-W clippy::pedantic`

warning: `pullpito` (lib) generated 1 warning (run `cargo clippy --fix --lib -p pullpito` to apply 1 suggestion)