Use co_withExecutor in folly/docs/examples/folly/ExecutorGuide.cpp +5

Summary:
`yourTask().scheduleOn(ex)` is deprecated in favor of `co_withExecutor(ex, yourTask())`

For `Task`, both forms are equivalent, but `co_withExecutor` makes it much easier to migrate to the [lifetime-safe `NowTask`](https://fb.prod.workplace.com/groups/192968587972839/posts/1784449738824708) (and upcoming `SafeTask`).

Thanks to [C++ ADL](https://en.cppreference.com/w/cpp/language/adl.html), you can write simply `co_withExecutor`; it does not *have* to be qualified with the namespace `folly::coro::`.

[This post](https://fb.prod.workplace.com/groups/192968587972839/posts/1693720697897613) explains the safety benefits of `co_withExecutor`. Moreover, the new name better aligns with the other `folly::coro` protocol of `co_withCancellation()`.

This diff generated with:
```
fbpython fbcode/scripts/rbarnes/schedule_on_fixer.py
```

 - If you approve of this diff, please use the "Accept & Ship" button :-)

Reviewed By: meyering

Differential Revision: D77751702

fbshipit-source-id: fda192e43473821ec7862e861323c217c8bedfdb

Author: r-barnes

third-party/folly/src/folly/docs/examples/folly/ExecutorGuide.cpp

@@ -66,7 +66,7 @@ int main() {
     folly::coro::Task t = foo(baton, 111);
 
     std::cout << "2. Assigning an Executor.\n";
-    folly::coro::TaskWithExecutor te = std::move(t).scheduleOn(executor);
+    folly::coro::TaskWithExecutor te = co_withExecutor(executor, std::move(t));
 
     std::cout << "3. Starting the Task.\n"
               << "   This is where the Coroutine calls executor.add().\n";