Use ? instead of unwrap in examples (#458)

This is recommended by the Rust API Guidelines:

https://rust-lang.github.io/api-guidelines/documentation.html#examples-use--not-try-not-unwrap-c-question-mark

One exception: When we need to unwrap an Option, the examples still use
.unwrap(). The alternative would be something like
`.ok_or(SomeErrorHere)?`, which feels like an awkward way to deal with
an Option. This might get better with NoneError:

https://docs.rs/rustc-std-workspace-std/1.0.1/std/option/struct.NoneError.html

I also rearranged some examples that used turbofish to use type
annotations. I think type annotations are more familiar to early Rust
users (and they use fewer characters and less punctuation, which is
always nice).

src/request.rs

@@ -424,7 +424,7 @@ impl Request {
     /// let req = ureq::get("http://httpbin.org/get")
     ///     .query("foo", "bar");
     ///
-    /// assert_eq!(req.request_url().unwrap().host(), "httpbin.org");
+    /// assert_eq!(req.request_url()?.host(), "httpbin.org");
     /// # Ok(())
     /// # }
     /// ```
@@ -490,7 +490,7 @@ impl RequestUrl {
     ///     .query("foo", "42")
     ///     .query("foo", "43");
     ///
-    /// assert_eq!(req.request_url().unwrap().query_pairs(), vec![
+    /// assert_eq!(req.request_url()?.query_pairs(), vec![
     ///     ("foo", "42"),
     ///     ("foo", "43")
     /// ]);