update merge failed msg (#83462)
Message seemed a bit incorrect to read
Ref: https://github.com/pytorch/pytorch/pull/82955#issuecomment-1215523319
Before PR:
```
Merge failed due to This PR is too stale; the last push date was more than 3 days ago. Please rebase and try again.
Raised by https://github.com/pytorch/pytorch/actions/runs/2862480424
```
After PR
```
Merge failed
Reason: This PR is too stale; the last push date was more than 3 days ago. Please rebase and try again.
Raised by https://github.com/pytorch/pytorch/actions/runs/2862480424
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83462
Approved by: https://github.com/janeyx99, https://github.com/ZainRizvi
