[css-fonts] Fix typo.

Author: fantasai

css-fonts-4/Overview.bs

@@ -773,7 +773,7 @@ If the 'font-min-size' property is computed to be larger than the 'font-max-size
 
 Users with accessibility preferences can set this in a user-agent style sheet to force websites' text to become larger.
 
-The computed value of these two properties affect the computed value of 'font-size'.
+The computed value of these two properties affects the computed value of 'font-size'.
 
 <h3 id="font-size-adjust-prop">
 Relative sizing: the 'font-size-adjust' property</h3>