Mention pytorchbot command on label error (#97267)
It's not clear how to add a label, especially for contributors without write permissions - they don't have a UI for that.
One recent struggle example - https://github.com/pytorch/pytorch/pull/94671
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97267
Approved by: https://github.com/PaliC, https://github.com/malfet, https://github.com/seemethere
