docs: remove extra backticks in `narrow_copy` (#54669)
Summary:
fixes https://github.com/pytorch/pytorch/issues/41590
https://11813004-65600975-gh.circle-artifacts.com/0/docs/tensors.html
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54669
Reviewed By: ailzhang
Differential Revision: D27328228
Pulled By: zou3519
fbshipit-source-id: 9a4a9bc4b265b0e82cf91f94dbbfd842fc42cdcb
