fix: Fix exclude (#537)

It wouldn't parse because it was missing the `//` before the EXCLUDE

src/Chapter13/Listing13.21.StaticAnonymousFunctions.cs

@@ -31,7 +31,7 @@ public static void Main()
                 // Error CS8820: A static anonymous function
                 // cannot contain a reference to comparisonCount.
                 comparisonCount++;
-                #endif // COMPILEERROR EXCLUDE
+                #endif // COMPILEERROR // EXCLUDE
                 return first < second;
             }
         );