Fix `serde::Deserialize` impl for `Url` prints confusing error message

When failing to deserialize a str into a Url the current implementation of Deserialize returns an error with a confusing message.

For example:

use serde_json::{json, Result};
use url::Url;

let json = json!("bad_url");
let result: Result<Url> = serde_json::from_value(json);
println!("{}", result.err().unwrap());

Actual: invalid value: string "bad_url", expected relative URL without a base
Expected: invalid value: string "bad_url", expected a string representing a URL

Locate the serde::Deserialize implementation for Url and reproduce the serde_json example from the issue to confirm the current error text. Update the reported expectation so invalid strings produce the clearer message, then run the relevant test suite and verify the example's output matches the expected wording.