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/response.rs

@@ -242,14 +242,15 @@ impl Response {
     ///
     /// ```
     /// use std::io::Read;
-    /// # fn main() -> Result<(), ureq::Error> {
+    /// # fn main() -> Result<(), Box<dyn std::error::Error>> {
     /// # ureq::is_test(true);
     /// let resp = ureq::get("http://httpbin.org/bytes/100")
     ///     .call()?;
     ///
     /// assert!(resp.has("Content-Length"));
-    /// let len = resp.header("Content-Length")
-    ///     .and_then(|s| s.parse::<usize>().ok()).unwrap();
+    /// let len: usize = resp.header("Content-Length")
+    ///     .unwrap()
+    ///     .parse()?;
     ///
     /// let mut bytes: Vec<u8> = Vec::with_capacity(len);
     /// resp.into_reader()
@@ -632,15 +633,18 @@ impl FromStr for Response {
     ///
     /// Example:
     /// ```
+    /// # fn main() -> Result<(), ureq::Error> {
     /// let s = "HTTP/1.1 200 OK\r\n\
     ///     X-Forwarded-For: 1.2.3.4\r\n\
     ///     Content-Type: text/plain\r\n\
     ///     \r\n\
     ///     Hello World!!!";
-    /// let resp = s.parse::<ureq::Response>().unwrap();
+    /// let resp: ureq::Response = s.parse()?;
     /// assert!(resp.has("X-Forwarded-For"));
-    /// let body = resp.into_string().unwrap();
+    /// let body = resp.into_string()?;
     /// assert_eq!(body, "Hello World!!!");
+    /// # Ok(())
+    /// # }
     /// ```
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         let stream = Stream::from_vec(s.as_bytes().to_owned());