Replace RegisterMessageHandler with RegisterMessageCallback in webui_explainer.md

It appears that RegisterMessageCallback is the correct method: https://source.chromium.org/chromium/chromium/src/+/master:content/public/browser/web_ui.h;drc=9d89d198cd0d8ad62d9cc1d6b3c60e29f98a3b0e;l=80
The only instance of RegisterMessageHandler is in that documentation page.

Change-Id: I98dbf30f4a12e25dd768687e396c96e0b16568d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626611
Auto-Submit: Kenton Lam <kentonlam@google.com>
Commit-Queue: dpapad <dpapad@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843163}

docs/webui_explainer.md

@@ -258,7 +258,7 @@ So, the given C++ code:
 
 ```c++
 void OvenHandler::RegisterMessages() {
-  web_ui()->RegisterMessageHandler("bakeDonuts",
+  web_ui()->RegisterMessageCallback("bakeDonuts",
       base::Bind(&OvenHandler::HandleBakeDonuts, base::Unretained(this)));
 }