Fix typo in add_pr_curve docstrings. (#49648)
Summary:
Very small PR to fix a typo.
### Description
Fixed 1 typo in the documentation of `torch/utils/tensorboard/writer.py` (replaced "_should in_" by "_should be in_")
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49648
Reviewed By: ngimel
Differential Revision: D25665831
Pulled By: mrshenli
fbshipit-source-id: a4e733515603bb9313c1267fdf2cfcc2bc2773c6
