Fix doctest better

Author: eholk

library/core/src/iter/sources/generator.rs

@@ -14,7 +14,8 @@
 ///     yield 1;
 ///     yield 2;
 ///     yield 3;
-/// } }();
+/// } };
+/// let mut it = it();
 /// let v: Vec<_> = it.collect();
 /// assert_eq!(v, [1, 2, 3]);
 /// ```