mkdir.rs: Fix a clippy warning on clippy::ptr-arg

[sylvestre]

error: writing `&mut Vec` instead of `&mut [_]` involves a new object where a slice will do
  --> src\uu\mkdir\src/mkdir.rs:72:33
   |
72 | fn strip_minus_from_mode(_args: &mut Vec<String>) -> bool {
   |                                 ^^^^^^^^^^^^^^^^ help: change this to: `&mut [String]`
   |
   = note: `-D clippy::ptr-arg` implied by `-D warnings`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg

[sylvestre]

trivial fix, autoapproving