Add missing inline code closure (#26239)

Author: adegeo

docs/csharp/whats-new/tutorials/nullable-reference-types.md

@@ -39,7 +39,7 @@ Create a new console application either in Visual Studio or from the command lin
 <Nullable>enable</Nullable>
 ```
 
-Prior to .NET 6, new projects do not include the `Nullable` element. Beginning with .NET 6, new projects include the `<Nullable>enable</Nullable> element in the project file.
+Prior to .NET 6, new projects do not include the `Nullable` element. Beginning with .NET 6, new projects include the `<Nullable>enable</Nullable>` element in the project file.
 
 ### Design the types for the application