Fix a formatting error

Author: DexterHaxxor

library/core/src/iter/traits/iterator.rs

@@ -803,7 +803,7 @@ pub trait Iterator {
     /// (0..5).map(|x| x * 2 + 1)
     ///       .for_each(move |x| tx.send(x).unwrap());
     ///
-    /// let v: Vec<_> =  rx.iter().collect();
+    /// let v: Vec<_> = rx.iter().collect();
     /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
     /// ```
     ///