Moves the Chalkboard DMI to the structures folder as opposed to 'structure'. (#4921)

## About The Pull Request

Having two folders, one being near empty is rather silly.

## Changelog
Not player facing.

modular_zubbers/code/game/objects/structures/chalkboard.dm

@@ -1,6 +1,6 @@
 /obj/structure/chalkboard
 	name = "Chalkboard"
-	icon = 'modular_zubbers/icons/obj/structure/chalkboard.dmi'
+	icon = 'modular_zubbers/icons/obj/structures/chalkboard.dmi'
 	icon_state = "chalkboard"
 	maptext_width = 64
 	//The text that is written on the chalkboard. Cleared when erased.