Inconsistency in README for version pinning

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## Describe the Bug
The README usage section says to use `version` to pin the atmos version, but the input section then gives `atmos-version` (which matches with `action.yml`). To make things more confusing, the [docs on the website](https://atmos.tools/integrations/github-actions/setup-atmos) say to use `atmos_version` (underscore instead of dash).

## Expected Behavior
Consistency in the documentation for input variables.

## Steps to Reproduce
Steps to reproduce the behavior:
1. Go to '...'
2. Run '....'
3. Enter '....'
4. See error

## Screenshots
If applicable, add screenshots or logs to help explain your problem.

## Environment (please complete the following information):

Anything that will help us triage the bug will help. Here are some ideas:
- OS: [e.g. Linux, OSX, WSL, etc]
- Version [e.g. 10.15]

## Additional Context
Add any other context about the problem here.