Remove trailing whitespace in iter chain doc

Author: pickfire

library/core/src/iter/adapters/chain.rs

@@ -12,7 +12,7 @@ use crate::usize;
 /// ```
 /// use std::iter::Chain;
 /// use std::slice::Iter;
-/// 
+///
 /// let a1 = [1, 2, 3];
 /// let a2 = [4, 5, 6];
 /// let iter: Chain<Iter<_>, Iter<_>> = a1.iter().chain(a2.iter());