Simplify doctests

src/digest.rs

@@ -183,18 +183,14 @@ impl Context {
 ///
 /// ```
 /// # #[cfg(feature = "use_heap")]
-/// # fn main() {
+/// # {
 /// use ring::{digest, test};
-///
 /// let expected_hex = "09ca7e4eaa6e8ae9c7d261167129184883644d07dfba7cbfbc4c8a2e08360d5b";
 /// let expected: Vec<u8> = test::from_hex(expected_hex).unwrap();
 /// let actual = digest::digest(&digest::SHA256, b"hello, world");
 ///
 /// assert_eq!(&expected, &actual.as_ref());
 /// # }
-///
-/// # #[cfg(not(feature = "use_heap"))]
-/// # fn main() { }
 /// ```
 pub fn digest(algorithm: &'static Algorithm, data: &[u8]) -> Digest {
     let mut ctx = Context::new(algorithm);