Update getting-started.rst

In the first example, the point that the test function and .py file should be prefixed or suffixed with 'test' is missing. It would be better if it is mentioned in the first example itself. So Added an extra phrase emphasizing to name the file as test_sample.py

doc/en/getting-started.rst

@@ -35,7 +35,7 @@ Install ``pytest``
 Create your first test
 ----------------------------------------------------------
 
-Create a simple test function with just four lines of code:
+Create a Python file test_sample.py and write a simple test function with just four lines of code:
 
 .. code-block:: python