Mention the 32x32 image requirement in the custom mouse cursor tutorial

This restriction has been increased (to 256x256) in master, but 3.0 still requires 32x32.

Related to godotengine/godot#17018

tutorials/inputs/custom_mouse_cursor.rst

@@ -20,7 +20,7 @@ Open project settings, go to Display>Mouse Cursor. You will see Custom Image and
 Custom Image is the desired image that you would like to set as the mouse cursor.
 Custom Hotspot is the point in the image that you would like to use as the cursor's detection point.
 
-.. note:: The custom image **must** be less than 256x256.
+.. note:: The custom image **must** be a 32x32. Any other size will not work.
 
 Using a script
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