Fix Utc example in README

[23ua]

Example of converting to Utc from the README doesn't compile:

use chrono::{TimeZone, UTC};
use chrono_tz::US::Pacific;

pub fn test() {
    let pacific_time = Pacific.ymd(1990, 5, 6).and_hms(12, 30, 45);
    let utc_time = pacific_time.with_timezone(&UTC);
    assert_eq!(utc_time, UTC.ymd(1990, 5, 6).and_hms(19, 30, 45));
}

error[E0432]: unresolved import `chrono::UTC`
 --> src/test.rs:1:24
  |
1 | use chrono::{TimeZone, UTC};
  |                        ^^^ no `UTC` in the root

error: aborting due to previous error

For more information about this error, try `rustc --explain E0432`.

This changes chrono::UTC -> chrono::Utc;