Fix a typo in Compute Layout and Allocation Size algorithm

Author: Djuffin

index.src.html

@@ -4118,7 +4118,7 @@
             multiplied by |sampleBytes|.
         9. Set |computedLayout|'s [=computed plane layout/sourceWidthBytes=] to
             the result of the integer division of
-            truncated |parsedRect|.{{DOMRectInit/width}} by |sampleHeight|,
+            truncated |parsedRect|.{{DOMRectInit/width}} by |sampleWidth|,
             multiplied by |sampleBytes|.
         10. If |layout| is not `undefined`:
             1. Let |planeLayout| be the {{PlaneLayout}} in |layout| at position