Fix markup in XML chapter

Author: javigomez

manual/en-US/coding-standards/chapters/xml.md

@@ -17,17 +17,17 @@ A max line length of 100 characters is recommended for good reading.
 #### Examples
 
 Element is **empty**:
-````
+```xml
 <field
 	name="abc"
 	type="text"
 	label="Empty Field"
 	description="Empty field without options"
 	/>
-````
+```
 
 Element is **not empty**:
-````
+```xml
 <field
 	name="abc"
 	type="radio"
@@ -54,4 +54,4 @@ Element is **not empty**:
 		SOMETHING_VERY_LONG
 	</option>
 </field>
-````
+```