Update index.html

Typo fix: let's -> lets

index.html

@@ -157,7 +157,7 @@ <h2>Transition Styles</h2>
 						<h2>Global State</h2>
 						<p>
 							Set <code>data-state="something"</code> on a slide and <code>"something"</code>
-							will be added as a class to the document element when the slide is open. This let's you
+							will be added as a class to the document element when the slide is open. This lets you
 							apply broader style changes, like switching the background.
 						</p>
 						<a href="#/7/1" class="image">