Fix incorrect reference to the 2D rendering context alpha flag

The steps of the 2D context creation algorithm erroneously
associated the alpha flag with the canvas element's bitmap.
The alpha flag actually belongs to the context.

source

@@ -59827,7 +59827,7 @@ interface <dfn>Path2D</dfn> {
 
      <dt><dfn><code data-x="dom-CanvasRenderingContext2DSettings-alpha">alpha</code></dfn></dt>
 
-     <dd>If false, then set the <var>target</var>'s bitmap's <span
+     <dd>If false, then set the new <code>CanvasRenderingContext2D</code> object's <span
      data-x="concept-canvas-alpha">alpha</span> flag to false.</dd>
 
     </dl>