Fix typo in Chapter 15

marijnh · marijnh · commit bed46bf741d5 · 2018-03-29T16:06:34.000+02:00
Closes marijnh#411
diff --git a/15_event.md b/15_event.md
@@ -939,7 +939,7 @@ document finishes loading, a `"load"` event fires on the window.
 
 {{index "balloon (exercise)", "arrow key"}}
 
-Write a page that display a ((balloon)) (using the balloon ((emoji)),
+Write a page that displays a ((balloon)) (using the balloon ((emoji)),
 🎈). When you press the up arrow, it should inflate (grow) ten percent,
 and when you press the down arrow, it should deflate (shrink) 10%.
 
