Update docs/tutorial/writing-tests/test-case.rst

Co-authored-by: Pierre Equoy <pierre.equoy@canonical.com>

docs/tutorial/writing-tests/test-case.rst

@@ -949,7 +949,7 @@ for exactly one situation, if possible. Consider the following:
    ``check_output_mock.assert_called_once()``. The reason is that we have to be
    compatible (in tests as well!) with python3.5 and
    ``Mock.assert_called_once`` was introduced in Python 3.6. If you don't know
-   when a function was introduced, refer to `the python documentation
+   when a function was introduced, refer to `the Python documentation
    <https://docs.python.org/3/library/unittest.mock.html#unittest.mock.Mock.assert_called_once>`_.
    See below the example, if a function was introduced with a specific
    version, you will find it there.