Don't link to specific WHATWG multipage page

"Links to the multipage version of the specification are unfortunately
likely to break over time."
-- https://html.spec.whatwg.org/multipage/asefij.html

This commit removes all references to the specific pages when viewing
WHATWG using multipage mode. I went through all these links and they
redirect fine.

Regex used to generate this commit:

`s_whatwg.org/multipage/.*#_whatwg.org/multipage/#_g`

components/canvas/canvas_paint_task.rs

@@ -92,7 +92,7 @@ impl<'a> CanvasPaintTask<'a> {
                     source_rect: Rect<f64>,
                     dest_rect: Rect<f64>,
                     smoothing_enabled: bool) {
-        // From spec https://html.spec.whatwg.org/multipage/scripting.html#dom-context-2d-drawimage
+        // From spec https://html.spec.whatwg.org/multipage/#dom-context-2d-drawimage
         // When scaling up, if the imageSmoothingEnabled attribute is set to true, the user agent should attempt
         // to apply a smoothing algorithm to the image data when it is scaled.
         // Otherwise, the image must be rendered using nearest-neighbor interpolation.