docs: Remove explicit auto deref from PrivateCookieJar example

[tyilo]

Motivation

Without it clippy produces the following lint:

warning: deref which would be done by auto-deref
   --> src/main.rs:101:9
    |
101 |         &*self.0
    |         ^^^^^^^^ help: try this: `&self.0`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#explicit_auto_deref
    = note: `#[warn(clippy::explicit_auto_deref)]` on by default

Solution

Trivial.

[davidpdrsn]

Thanks!