fountain/doctest: simplify xor function

src/fountain.rs

@@ -8,11 +8,11 @@
 //! A seeded `Xoshiro` RNG ensures that the receiver can reconstruct which segments
 //! were combined into the part.
 //! ```
-//! let xor = |a: &[u8], b: &[u8]| {
+//! let xor = |a: &[u8], b: &[u8]| -> Vec<_> {
 //!     a.iter()
 //!         .zip(b.iter())
-//!         .map(|(&x1, &x2)| x1 ^ x2)
-//!         .collect::<Vec<_>>()
+//!         .map(|(x1, x2)| x1 ^ x2)
+//!         .collect()
 //! };
 //!
 //! let data = String::from("Ten chars!");