Correct invariant documentation for steps_between

Given that the previous example involves stepping forward from A to B,
the equivalent example on this line would make most sense as stepping
backward from B to A.

Author: dhwthompson

library/core/src/iter/range.rs

@@ -30,7 +30,7 @@ pub trait Step: Clone + PartialOrd + Sized {
     /// For any `a`, `b`, and `n`:
     ///
     /// * `steps_between(&a, &b) == Some(n)` if and only if `Step::forward_checked(&a, n) == Some(b)`
-    /// * `steps_between(&a, &b) == Some(n)` if and only if `Step::backward_checked(&a, n) == Some(a)`
+    /// * `steps_between(&a, &b) == Some(n)` if and only if `Step::backward_checked(&b, n) == Some(a)`
     /// * `steps_between(&a, &b) == Some(n)` only if `a <= b`
     ///   * Corollary: `steps_between(&a, &b) == Some(0)` if and only if `a == b`
     ///   * Note that `a <= b` does _not_ imply `steps_between(&a, &b) != None`;