Fix Atis Tutorial Reference in Docstring

[m3rlin45]

Summary:
The reference to the atis tutorial in model.py doesn't work properly and is throwing a warning. This fixes it.
Test Plan:
make clean-all
make html
then validated the warning went away

existing docs look like:
https://pytext-pytext.readthedocs-hosted.com/en/latest/modules/pytext.models.html (grep for atis)
new ones look like (from CI build) https://834-143080897-gh.circle-artifacts.com/0/docs/modules/pytext.models.html
as you can see, the reference is nicely expanded

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

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[facebook-github-bot]

@bethebunny is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.