fix: typo in lesson 18 (#577)

Author: pcamp

course/lesson-18-for-loops/lesson.tres

@@ -134,7 +134,7 @@ script = ExtResource( 3 )
 content_id = "res://course/lesson-18-for-loops/content-izKUdOCQ.tres"
 title = "Using a for loop instead of a while loop"
 type = 0
-text = "Here's our old [code]move_to_end()[/code] function which used in a [code]while[/code] loop."
+text = "Here's our old [code]move_to_end()[/code] function which used a [code]while[/code] loop."
 visual_element_path = ""
 reverse_blocks = false
 has_separator = false