Fix layer norm documentation

[eric-haibin-lin]

Description

The documentation of nn.LayerNorm is incorrect in terms of how eps is used. Update it so that it's now consistent with ndarray.LayerNorm.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• Changes are complete (i.e. I finished coding on this PR)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http:https://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

[sxjscience]

Thanks for the fix!

[eric-haibin-lin]

@marcoabreu @larroy do you know why ci/jenkins/mxnet-validation/edge is taking forever for this simple change?

[szha]

Is the mshadow change intentional?

[eric-haibin-lin]

@szha No that's an accident when I tried to trigger CI. I'll force push again