Fix code block not having XML syntax highlighting

Signed-off-by: Indicado <[email protected]>

docs/guides/xml-pointers/conventions.mdx

@@ -35,7 +35,7 @@ Modules that have sub elements such as the `author` tag must be indented with 4
 
 There should be no white spaces between modules. That means the end of one module should have the start of another on the immediate next line.
 
-```
+```xml
 <!-- Correct -->
 <?xml version="1.0"?>
 <map proto="1.4.0">