📝 Fix outdated on_event reference in async tests docs

The async tests page told readers to instantiate event-loop-bound
objects inside an `@app.on_event("startup")` callback, but `on_event`
is deprecated in favor of `lifespan` (as documented on the Lifespan
Events page, and as this same page already references a few lines
above when discussing lifespan events with the AsyncClient).

docs/en/docs/advanced/async-tests.md

@@ -94,6 +94,6 @@ As the testing function is now asynchronous, you can now also call (and `await`)
 
 /// tip
 
-If you encounter a `RuntimeError: Task attached to a different loop` when integrating asynchronous function calls in your tests (e.g. when using [MongoDB's MotorClient](https://stackoverflow.com/questions/41584243/runtimeerror-task-attached-to-a-different-loop)), remember to instantiate objects that need an event loop only within async functions, e.g. an `@app.on_event("startup")` callback.
+If you encounter a `RuntimeError: Task attached to a different loop` when integrating asynchronous function calls in your tests (e.g. when using [MongoDB's MotorClient](https://stackoverflow.com/questions/41584243/runtimeerror-task-attached-to-a-different-loop)), remember to instantiate objects that need an event loop only within async functions, e.g. an async `lifespan` handler.
 
 ///