Doc: update dev/design/pdftheory with new function call

NOTRY=true
TBR=

Review URL: https://codereview.chromium.org/967873002

site/dev/design/pdftheory.md

@@ -56,8 +56,9 @@ longer document with a bunch of other pages.
 
 <!--?prettify lang=cc?-->
 
+    SkPDFCanon canon;
     SkAutoUnref<SkPDFDevice> pdfDevice(
-        new SkPDFDevice(width, height, initial_transform));
+        SkPDFDevice::Create(SkISize::Make(width, height), 72.0f, &canon));
 
     SkCanvas canvas(pdfDevice);
     draw_content(&canvas);