Fix typo in docstring (#2052)

Author: temoto

src/prompt_toolkit/application/application.py

@@ -920,7 +920,7 @@ def run(
             thread, and that loop will also be closed when the background
             thread terminates. When this is used, it's especially important to
             make sure that all asyncio background tasks are managed through
-            `get_appp().create_background_task()`, so that unfinished tasks are
+            `get_app().create_background_task()`, so that unfinished tasks are
             properly cancelled before the event loop is closed. This is used
             for instance in ptpython.
         :param handle_sigint: Handle SIGINT signal. Call the key binding for