Fix duplicated warning for rand::Rng::gen_ascii_chars

[kngwyu]

Fix

warning: use of deprecated item 'rand::Rng::gen_ascii_chars': use sample_iter(&Alphanumeric) instead
   --> src/actions/format.rs:120:39
    |
120 |     let suffix: String = thread_rng().gen_ascii_chars().take(SUFFIX_LEN).collect();
    |                                       ^^^^^^^^^^^^^^^
    |
    = note: #[warn(deprecated)] on by default

[Xanewok]

Thanks! =)