[mojo][doc] Update the security tips page to use pending_remote

Change-Id: Ie3133d8a9d1b9887265639bea52066cf9c8cf869
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2146112Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758363}

docs/security/mojo.md

@@ -22,7 +22,7 @@ that needs to be maintained in sync.
 
 ```c++
 interface TeleporterFactory {
-  Create(Location start, Location end) => (Teleporter);
+  Create(Location start, Location end) => (pending_remote<Teleporter>);
 };
 
 interface Teleporter {