Updating out of date documentation in WebUI Explainer

In the instructions of how the browser (c++) and Renderer (JS)
communicate, there is a step to register the mojo PageHandlerFactory to
your own controller. That step explained to do that in
chrome/browser/chrome/browser_interface_binders.cc, however all of the
registrations now exist in
chrome/browser/chrome_browser_interface_binders_webui.cc

This CL updates the doc with the correct path to register the Mojo
PageHandlerFactory to your own controller.

Change-Id: Ibdbb3041961a555fac044c6090f965850ade2b4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6371454
Reviewed-by: Elly FJ <ellyjones@chromium.org>
Commit-Queue: Jacob Stanley <jacobstanley@google.com>
Auto-Submit: Jacob Stanley <jacobstanley@google.com>
Cr-Commit-Position: refs/heads/main@{#1434902}

docs/webui/webui_explainer.md

@@ -586,7 +586,7 @@ void DonutsUI::CreatePageHandler(
 ```
 
 You also need to register the PageHandlerFactory to your controller in
-**chrome/browser/chrome_browser_interface_binders.cc**:
+**chrome/browser/chrome_browser_interface_binders_webui.cc**:
 ```c++
 RegisterWebUIControllerInterfaceBinder<donuts::mojom::PageHandlerFactory,
                                        DonutsUI>(map);