[css-scroll-snap-1] Fix computed value lines of scroll-margin to not imply that it takes a percentage. w3c#3289

Author: fantasai

css-scroll-snap-1/Overview.bs

@@ -514,7 +514,7 @@ Scroll Snapping Area: the 'scroll-margin' property {#scroll-margin}
     Applies to: all elements
     Inherited: no
     Percentages: n/a
-    Computed value: per side, a computed <<length-percentage>> value
+    Computed value: per side, an absolute length
     Animation type: by computed value type
     </pre>
 
@@ -1041,7 +1041,7 @@ Physical Longhands for 'scroll-margin'  {#margin-longhands-physical}
     Applies to: all elements
     Inherited: no
     Percentages: n/a
-    Computed value: a computed <<length-percentage>> value
+    Computed value: absolute length
     Animation type: by computed value type
     </pre>
 
@@ -1059,7 +1059,7 @@ Flow-relative Longhands for 'scroll-margin'  {#margin-longhands-logical}
     Applies to: all elements
     Inherited: no
     Percentages: n/a
-    Computed value: a computed <<length-percentage>> value
+    Computed value: absolute length
     Animation type: by computed value type
     </pre>