Fix code block

Author: sbomer

xml/System.Diagnostics.CodeAnalysis/FeatureSwitchDefinitionAttribute.xml

@@ -51,9 +51,11 @@ public class Feature
 
     internal static Implementation() => ...;
 }
-`
+```
+
 When the app is trimmed with the following feature settings in the project file,
 `Feature.IsSupported` is treated as `false`, and `Feature.Implementation` code is removed.
+
 ```xml
 <ItemGroup>
   <RuntimeHostConfigurationOption Include="Feature.IsSupported" Value="false" />