Add missing assertion

Author: vcfxb

library/core/src/bool.rs

@@ -58,6 +58,7 @@ impl bool {
     ///
     /// // `a` is incremented once because the closure is evaluated lazily by
     /// // `then`.
+    /// assert_eq!(a, 1);
     /// ```
     #[stable(feature = "lazy_bool_to_option", since = "1.50.0")]
     #[rustc_const_unstable(feature = "const_bool_to_option", issue = "91917")]