Fix paths for pytest in contribution guide (#20655)

Author: mmicu

CONTRIBUTING.md

@@ -162,25 +162,25 @@ We use [pytest](https://pytest.org/) to run the tests.
 
 ### Run a test file
 
-To run the tests in `keras/losses/losses_test.py`, use the following command
+To run the tests in `keras/src/losses/losses_test.py`, use the following command
 at the root directory of the repo.
 
 ```shell
-pytest keras/losses/losses_test.py
+pytest keras/src/losses/losses_test.py
 ```
 
 ### Run a single test case
 
 You can specify a single test class to run within a file.
 
 ```shell
-pytest keras/losses/losses_test.py::MeanSquaredErrorTest
+pytest keras/src/losses/losses_test.py::MeanSquaredErrorTest
 ```
 
 You can also specify a single test method to run within a class.
 
 ```shell
-pytest keras/losses/losses_test.py::MeanSquaredErrorTest::test_sample_weighted
+pytest keras/src/losses/losses_test.py::MeanSquaredErrorTest::test_sample_weighted
 ```
 
 ### Run all tests