[WebuiExplainer] Update documentation with a missing mojo dependency

I cam across this missing dependency when implementing
https://crrev.com/c/5817454.

Change-Id: I0d430949c5ea7a7e8bc611b0a94049400ff0e0b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5857164
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Ryan Sultanem <rsult@google.com>
Cr-Commit-Position: refs/heads/main@{#1360594}

docs/webui_explainer.md

@@ -685,6 +685,8 @@ build_webui("build") {
     "browser_proxy.ts",
   ]
 
+  ts_deps = [ "//ui/webui/resources/mojo:build_ts" ]
+
   mojo_files_deps =
       [ "//chrome/browser/ui/webui/donuts:mojo_bindings_ts__generator" ]
   mojo_files = [