fix trailing whitespace

Author: LFS6502

library/alloc/src/vec/mod.rs

@@ -3046,7 +3046,7 @@ impl<T: Clone, A: Allocator> Vec<T, A> {
 
     /// let mut characters3 = vec!['a', 'b', 'c', 'd', 'e'];
     /// characters2.extend_from_within(1..3);
-    /// assert_eq!(characters3, ['a', 'b', 'c', 'd', 'e', 'b', 'c']);   
+    /// assert_eq!(characters3, ['a', 'b', 'c', 'd', 'e', 'b', 'c']);
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "vec_extend_from_within", since = "1.53.0")]