content: clarify requirements in lesson 21 combo practice

fix #708

course/lesson-21-strings/lesson.tres

@@ -177,7 +177,7 @@ Populate the combo array with animation names as strings.
 
 Then, for each action in the array, call the [code]play_animation()[/code] function to play them.
 
-The robot should do the following actions in order: [code]jab, jab, uppercut[/code]."
+The array should contain three values, so the robot makes these three attacks: two jabs followed by one uppercut."
 starting_code = "func run():
 	combo = []"
 cursor_line = 0