[selectors-4] Fix typo (#1886)

Author: Kristofor Salmin

selectors-4/Overview.bs

@@ -2061,7 +2061,7 @@ The Input Focus-Ring Pseudo-class: '':focus-ring''</h3>
 
 		On the other hand, they typically only display focus indicators on buttons
 		when they were focused by a keyboard interaction
-		(such as tabbing thru the document),
+		(such as tabbing through the document),
 		because it's not always immediately obvious where the focus will move
 		after such an interaction,
 		but not when they were focused by more "obvious" interactions,