WARN fixup warning in pandas/tests/apply/test_frame_apply.py

[MarcoGorelli]

We're not (yet) running all tests with -W error, so some warnings slip through to CI. On the way towards that, let's fixup this file.

Task here is:

1. make sure you've got your environment set up, see the contributing guide
2. make sure you can run this test: pytest pandas/tests/apply/test_frame_apply.py
3. fixup the warning (you just need to add dayfirst=True to the to_datetime call)
4. check that pytest pandas/tests/apply/test_frame_apply.py doesn't throw a warning
5. stage, commit, push, open pull request, celebrate 🎉

Feel free to tag me for review

[tpackard1]

take