make group example more idiomatic
//!
//! ```ignore
//! stream.group(|key, vals, output| {
-//! let (mut max_val, mut max_wgt) = vals.next().unwrap();
-//! for (val, wgt) in vals {
-//! if wgt > max_wgt {
-//! max_wgt = wgt;
-//! max_val = val;
-//! }
-//! }
-//! output.push((max_val.clone(), max_wgt));
+//! output.push(vals.iter().max_by_key(|&(_val, wgt)| wgt).unwrap());
//! })
//! ```
