Fix warning about hidden elided lifetime in Drain

[niklasf]

warning: hiding a lifetime that's elided elsewhere is confusing
   --> src/arrayvec.rs:640:21
    |
640 |     pub fn drain<R>(&mut self, range: R) -> Drain<T, CAP>
    |                     ^^^^^^^^^               ^^^^^^^^^^^^^ the same lifetime is hidden here
    |                     |
    |                     the lifetime is elided here
    |
    = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
    = note: `#[warn(mismatched_lifetime_syntaxes)]` on by default
help: use `'_` for type paths
    |
640 |     pub fn drain<R>(&mut self, range: R) -> Drain<'_, T, CAP>
    |                                                   +++