Fix xml syntax in shadow texture size tutorial (#3111)

Signed-off-by: Ian Chen <ichen@openrobotics.org>

Author: iche033

tutorials/shadow_texture_size.md

@@ -100,7 +100,7 @@ Let's change the shadow texture size for directional light with the SDF file bel
 2) Add the following lines to the Minimal Scene GUI plugin.
 
 ```xml
-</shadows>
+<shadows>
   <texture_size light_type="directional">8192</texture_size>
 </shadows>
 ```