Add type hint to background-size page

Co-authored-by: wongjn <11310624+wongjn@users.noreply.github.com>

Author: philipp-spiess

src/docs/background-size.mdx

@@ -93,7 +93,7 @@ Use the `bg-auto` utility to display the background image at its default size:
 
 ### Using a custom value
 
-<UsingACustomValue utility="bg" value="200px_100px" name="background image size" variable="image-size" />
+<UsingACustomValue utility="bg" value="50%_auto" name="background image size" variable="image-size" dataType="length" />
 
 ### Responsive design