Update app/code/Magento/ConfigurableProduct/README.md

MC-40719: Product Thumbnails not switching properly for Config product

Co-authored-by: Jeff Matthews <matthews.jeffery@gmail.com>

Author: thiaramus

app/code/Magento/ConfigurableProduct/README.md

@@ -39,7 +39,7 @@ If the `gallery_switch_strategy` variable is not defined, the default value `rep
 
 For example, adding these lines of code to the theme view.xml will set the gallery behavior to `replace` mode. 
 
-```
+```xml
 <vars module="Magento_ConfigurableProduct">
     <var name="gallery_switch_strategy">replace</var>
 </vars>