Add example (#24406)

* Add example

Added an example of how to use is keyword

* Fixed error in syntax highlighting

guide/english/csharp/is/index.md

@@ -5,5 +5,13 @@ title: is
 ## is
 The `is` keyword checks if an object is compatible with a given type, or (starting with C# 7) tests an expression against a pattern.
 
+## Example
+```csharp
+  int number = 6;
+  Console.WriteLine(number is long);    // False
+  Console.WriteLine(number is double);  // False
+  Console.WriteLine(number is int);     // True
+```
+
 #### More information:
 - [C# Reference: is](https://docs.microsoft.com/dotnet/csharp/language-reference/keywords/is)