Fix typo in ChainedScheduler docstring (#79775)
### Goal
Fixes https://github.com/pytorch/pytorch/issues/79720
### Approach
replace `Chains list of learning rate schedulers. It takes a list of chainable learning rate schedulers and performs consecutive step() functions` **`belong`** `to them by just one call.` with `Chains list of learning rate schedulers. It takes a list of chainable learning rate schedulers and performs consecutive step() functions` **`belonging`** `to them by just one call.`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79775
Approved by: https://github.com/albanD
