Fixed broken link (Res-nonnegative) (#1641)

Author: morinim

CppCoreGuidelines.md

@@ -1604,7 +1604,7 @@ Once language support becomes available (e.g., see the [contract proposal](http:
 
 ##### Note
 
-No, using `unsigned` is not a good way to sidestep the problem of [ensuring that a value is non-negative](#Res-non-negative).
+No, using `unsigned` is not a good way to sidestep the problem of [ensuring that a value is non-negative](#Res-nonnegative).
 
 ##### Enforcement