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/error.rs

@@ -116,6 +116,7 @@ impl Error {
 /// * [`Transport::source()`](std::error::Error::source) holds the underlying error with even more details.
 ///
 /// ```
+/// # fn main() -> Result<(), Box<dyn std::error::Error>> {
 /// use ureq::ErrorKind;
 /// use std::error::Error;
 /// use url::ParseError;
@@ -137,9 +138,11 @@ impl Error {
 /// // boxed underlying error
 /// let source = error.source().unwrap();
 /// // downcast to original error
-/// let downcast = source.downcast_ref::<ParseError>().unwrap();
+/// let downcast: &ParseError = source.downcast_ref().unwrap();
 ///
 /// assert_eq!(downcast.to_string(), "relative URL without a base");
+/// # Ok(())
+/// # }
 /// ```
 #[derive(Debug)]
 pub struct Transport {