[package] Minor fixes to PackageExporter docstrings (#55817)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55817
**Summary**
This commit makes minor edits to the docstrings of `PackageExporter` so
that they render properly in the `torch.package` API reference.
**Test Plan**
Continuous integration (especially the docs tests).
Test Plan: Imported from OSS
Reviewed By: gmagogsfm
Differential Revision: D27726817
Pulled By: SplitInfinity
fbshipit-source-id: b81276d7278f586fceded83d23cb4d0532f7c629
