fix example with an invalid preprocessor expression (#40857)

elif directive without a condition

docs/csharp/language-reference/preprocessor-directives.md

@@ -88,12 +88,12 @@ The following code is compiled when `MYTEST` is **not** defined:
 
 You can use the operators [`==` (equality)](operators/equality-operators.md#equality-operator-) and [`!=` (inequality)](operators/equality-operators.md#inequality-operator-) to test for the [`bool`](builtin-types/bool.md) values `true` or `false`. `true` means the symbol is defined. The statement `#if DEBUG` has the same meaning as `#if (DEBUG == true)`. You can use the [`&&` (and)](operators/boolean-logical-operators.md#conditional-logical-and-operator-), [`||` (or)](operators/boolean-logical-operators.md#conditional-logical-or-operator-), and [`!` (not)](operators/boolean-logical-operators.md#logical-negation-operator-) operators to evaluate whether multiple symbols have been defined. You can also group symbols and operators with parentheses.
 
-The following is a complex directive that allows your code to take advantage of newer .NET features while remaining backward compatible.  For example, imagine that you're using a NuGet package in your code, but the package only supports .NET 6 and up, as well as .NET Standard 2.0 and up:
+The following is a complex directive that allows your code to take advantage of newer .NET features while remaining backward compatible. For example, imagine that you're using a NuGet package in your code, but the package only supports .NET 6 and up, as well as .NET Standard 2.0 and up:
 
 ```csharp
 #if (NET6_0_OR_GREATER || NETSTANDARD2_0_OR_GREATER)
     Console.WriteLine("Using .NET 6+ or .NET Standard 2+ code.");
-#elif
+#else
     Console.WriteLine("Using older code that doesn't support the above .NET versions.");
 #endif
 ```