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/lib.rs

@@ -263,7 +263,7 @@
 //!   use ureq::Agent;
 //!
 //!   let agent = ureq::AgentBuilder::new()
-//!       .tls_connector(Arc::new(native_tls::TlsConnector::new().unwrap()))
+//!       .tls_connector(Arc::new(native_tls::TlsConnector::new()?))
 //!       .build();
 //! # Ok(())
 //! # }
@@ -487,7 +487,7 @@ pub fn request(method: &str, path: &str) -> Request {
 /// use url::Url;
 /// let agent = ureq::agent();
 ///
-/// let mut url: Url = "http://example.com/some-page".parse().unwrap();
+/// let mut url: Url = "http://example.com/some-page".parse()?;
 /// url.set_path("/robots.txt");
 /// let resp: ureq::Response = ureq::request_url("GET", &url)
 ///     .call()?;