ES.30 fix example code

[SpeedReach]

The example should be passing in a constexpr and validating it at compile time like a macro, but instead passes a unknown value at compile time. Which may cause mislead and the code doesn't even work.

Ref: #2136

[hsutter]

Editors call: We agree the example is incorrect and want to adjust the proposed solution.

[SpeedReach]

Let me know if there's any change needed in the pull request, thanks!

[cubbimew]

I think the main reaction was that use of numbers made it harder to quickly grasp and the out-of-range constant was distracting, how about make it go like this:

        string s1 = stringify<a>(); 
        string s2 = stringify<b>(); 

[SpeedReach]

Sure! The change is done in the latest commit.

[cubbimew]

thanks. lgtm