Casting away const isn't undefined behavior, but modifying a constant is (#1509)

* Casting away const isn't undefined behavior, but modifying a constant is

You can cast away `const` as much as you like, as long as you never write to variable.

* rewording for clarification

Author: hsutter

CppCoreGuidelines.md

@@ -11780,7 +11780,7 @@ for example.)
 ##### Reason
 
 It makes a lie out of `const`.
-If the variable is actually declared `const`, the result of "casting away `const`" is undefined behavior.
+If the variable is actually declared `const`, modifying it results in undefined behavior.
 
 ##### Example, bad