ES.30 fix example code (#2143)

* ES.30 fix example code 

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

* Update CppCoreGuidelines.md

Used enums instead of raw numbers.

CppCoreGuidelines.md

@@ -11368,8 +11368,6 @@ Also, `#` and `##` encourages the definition and use of macros:
 
 There are workarounds for low-level string manipulation using macros. For example:
 
-    string s = "asdf" "lkjh";   // ordinary string literal concatenation
-
     enum E { a, b };
 
     template<int x>
@@ -11381,9 +11379,10 @@ There are workarounds for low-level string manipulation using macros. For exampl
         }
     }
 
-    void f(int x, int y)
+    void f()
     {
-        string sx = stringify<x>();
+        string s1 = stringify<a>();
+        string s2 = stringify<b>();
         // ...
     }