Grammatically update tech docs (#54370)
Summary:
Small grammatical update to nn.rst
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54370
Reviewed By: radkris-git
Differential Revision: D27243944
Pulled By: heitorschueroff
fbshipit-source-id: 08d8061d9e74ffaf95c8a610107a8632259474ca
