[css-scroll-snap-1] Fix scroll-padding-inline/block grammar ad hope ewilligers don't hate me yet for screwing this up so many times. w3c#3189

Author: fantasai

css-scroll-snap-1/Overview.bs

@@ -1015,7 +1015,7 @@ Flow-relative Longhands for 'scroll-padding'  {#padding-longhands-logical}
 
     <pre class="propdef shorthand">
     Name: scroll-padding-block, scroll-padding-inline
-    Value: auto | <<length-percentage>>{1,2}
+    Value: [ auto | <<length-percentage>> ]{1,2}
     Initial: auto
     Applies to: all elements
     Inherited: no