ipnet: remove unneeded ref to make clippy happy

[taspelund]

Clippy whining:

treyaspelund@Tallon-IV ~/git/oxnet xyoy 25cc (empty) (no description)
› cargo clippy --all-targets -- --deny warnings
    Checking oxnet v0.1.7 (/Users/treyaspelund/git/oxnet)
error: redundant reference in `write!` argument
   --> src/ipnet.rs:652:28
    |
652 |         write!(f, "{}/{}", &self.addr, self.width)
    |                            ^^^^^^^^^^ help: remove the redundant `&`: `self.addr`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.98.0/index.html#useless_borrows_in_formatting
    = note: `-D clippy::useless-borrows-in-formatting` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::useless_borrows_in_formatting)]`

error: redundant reference in `write!` argument
    --> src/ipnet.rs:1056:28
     |
1056 |         write!(f, "{}/{}", &self.addr, self.width)
     |                            ^^^^^^^^^^ help: remove the redundant `&`: `self.addr`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.98.0/index.html#useless_borrows_in_formatting

error: could not compile `oxnet` (lib) due to 2 previous errors
warning: build failed, waiting for other jobs to finish...
error: could not compile `oxnet` (lib test) due to 2 previous errors