Fix String implementation for Deref explanation

[danielthank]

deref() returns &Target

pub trait Deref {
    type Target: ?Sized;

    // Required method
    fn deref(&self) -> &Self::Target;
}

So Target should be str instead of &str for String type

Deref trait source code: https://doc.rust-lang.org/src/core/ops/deref.rs.html#139-151
String implementation source code: https://doc.rust-lang.org/src/alloc/string.rs.html#2722-2730

[YHTerrance]

Looks good to me! Thanks for the correction.