updated outdated code sample in the readme

[x] I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
[ ] I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
[ ] I have properly documented new or changed features in the documentation or in the docstrings
[ ] I have properly explained unusual or unexpected code in the comments around it

Zulko / moviepy