content: Improve lesson 15 note title (#740)

Author: assertchris

course/lesson-15-modulo/lesson.tres

@@ -111,7 +111,7 @@ has_separator = false
 [sub_resource type="Resource" id=9]
 script = ExtResource( 5 )
 content_id = "res://course/lesson-15-modulo/content-x0c7DDiz.tres"
-title = ""
+title = "Why do we start from zero?"
 type = 1
 text = "In computer code, we very often count from [code]0[/code].