doc: Fix a link to test/lint/lint-python.py

Author: hebasto

test/functional/README.md

@@ -24,7 +24,7 @@ don't have test cases for.
   Consider using [pyenv](https://github.com/pyenv/pyenv), which checks [.python-version](/.python-version),
   to prevent accidentally introducing modern syntax from an unsupported Python version.
   The CI linter job also checks this, but [possibly not in all cases](https://github.com/bitcoin/bitcoin/pull/14884#discussion_r239585126).
-- See [the python lint script](/test/lint/lint-python.sh) that checks for violations that
+- See [the python lint script](/test/lint/lint-python.py) that checks for violations that
   could lead to bugs and issues in the test code.
 - Use [type hints](https://docs.python.org/3/library/typing.html) in your code to improve code readability
   and to detect possible bugs earlier.