Correct OneCycleLR doc example code to explicitly call optimizer.step() (#95730)
Fixes #89358 as suggested in the issue comment
A screenshot of the example code in the built docs:
<img width="1168" alt="Screenshot 2023-02-28 at 4 46 45 PM" src="https://user-images.githubusercontent.com/31816267/221999156-02b28f2a-85b3-4aa8-841d-e4c66a39a33f.png">
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95730
Approved by: https://github.com/janeyx99
