fix broken links in testing.mdx (#19820)

docs/source/en/testing.mdx

@@ -514,7 +514,7 @@ spawns a normal process that then spawns off multiple workers and manages the IO
 
 Here are some tests that use it:
 
-- [test_trainer_distributed.py](https://github.com/huggingface/transformers/tree/main/tests/test_trainer_distributed.py)
+- [test_trainer_distributed.py](https://github.com/huggingface/transformers/tree/main/tests/trainer/test_trainer_distributed.py)
 - [test_deepspeed.py](https://github.com/huggingface/transformers/tree/main/tests/deepspeed/test_deepspeed.py)
 
 To jump right into the execution point, search for the `execute_subprocess_async` call in those tests.