[css-fonts-5] Don't allow more than 2 values for the font-size @font-face descriptor

Author: litherum

css-fonts-5/Overview.bs

@@ -119,7 +119,7 @@ Issue(5892):
 
 <pre class='descdef'>
     Name: font-size
-    Value: auto | [<<number>>]#
+    Value: auto | <<number>> <<number>>?
     Initial: normal
     For: @font-face
     </pre>