RangeFrom::step_by docs: fix example

The previous example did not do what its description said. In it panicked on integer overflow in debug mode, and went into an infinite loop in release mode (wrapping back to 0 after printing 254).

Author: SimonSapin

src/libcore/iter.rs

@@ -4252,13 +4252,15 @@ impl<A: Step> RangeFrom<A> {
     ///
     /// # Examples
     ///
-    /// ```ignore
-    /// for i in (0u8..).step_by(2) {
+    /// ```
+    /// # #![feature(step_by)]
+    ///
+    /// for i in (0u8..).step_by(2).take(10) {
     ///     println!("{}", i);
     /// }
     /// ```
     ///
-    /// This prints all even `u8` values.
+    /// This prints the first ten even natural integers (0 to 18).
     #[unstable(feature = "step_by", reason = "recent addition",
                issue = "27741")]
     pub fn step_by(self, by: A) -> StepBy<A, Self> {